In advanced English syntax and subject-verb concord, navigating complex structural anomalies requires precise application of proximity agreement, inverse copular structures, inverted subjects, and quantified nominal complexes. Consider the following four advanced propositions regarding subject-verb agreement: I. In an inverse copular sentence where the subjective complement and the referential subject differ in grammatical number (e.g., 'The primary cause of the systemic economic collapse was the uncontrolled speculative investments'), prescriptive syntax dictates that the finite verb must agree with the post-copular noun phrase acting as the true subject rather than the pre-copular predicative complement. II. When a plural noun phrase expressing a specific monetary budget, distance, or aggregate quantity is preceded by restrictive modifiers or appears in inverted existential structures (e.g., 'There remain only fifty dollars in the emergency reserve fund'), the dummy pronoun 'there' governs the verb number, ensuring the finite verb remains plural to match the logical subject. III. When a relative pronoun ('who', 'which', or 'that') functions as the subject of a relative clause, the number and person of the finite verb within that clause are determined strictly by the grammatical antecedent of the relative pronoun, superseding any intervening nominal distractors (e.g., 'It is one of those complex administrative policies that require immediate revision'). IV. When a compound subject consists of coordinated nominals joined by 'neither... nor' or 'either... or' where the pronouns are of mixed grammatical person and number (e.g., 'Neither the lead investigator nor the assistants was present at the scene'), standard formal prescriptive grammar mandates that the verb must default to a plural form by overriding proximity rules entirely. Identify the option that correctly evaluates the grammatical, syntactic, and semantic validity of propositions I, II, III, and IV according to standard English grammar and Agreement (linguistics) principles.

Let us analyze each proposition in detail based on advanced principles of English syntax and Agreement (linguistics): - **Proposition I is CORRECT.** In inverse copular sentences, the finite copular verb ("was") must agree with the true referential subject ("uncontrolled speculative investments", which is plural) rather than the pre-copular predicative nominal ("The primary cause", which is singular). Although traditional prescriptive usage sometimes favored agreement with the pre-copular complement, modern formal syntax dictates that the verb agrees with the subject complement in specific reverse copular constructions to reflect true underlying thematic structure. - **Proposition II is INCORRECT.** In existential "there" constructions, "there" is merely an expletive dummy subject that occupies the subject slot for word-order requirements, but the true grammatical subject follows the verb. Therefore, the verb's number is controlled by the following post-verbal noun phrase. In the example "There remain only fifty dollars...", "fifty dollars" is treated here as a plural set of discrete currency units or simply matches the plural noun phrase; however, when treated as a single unified amount (e.g., "There is fifty dollars in the drawer"), a singular verb is standard. More importantly, dummy "there" does not possess inherent number; the verb agrees strictly with the following nominal head ("fifty dollars", which governs a plural verb when viewed as individual units, but the proposition misstates the general existential rule where mass/aggregate sums take singular verbs). - **Proposition III is CORRECT.** This is a classic advanced concord rule. In sentences of the structure *[It is one of those N + relative pronoun + verb]* (often called *one of those who* constructions), the relative pronoun ("that") refers back to the plural noun ("policies"), not to "one". Therefore, the relative clause verb must be plural ("require"). If the structure were modified with *only* (*It is the only one of those policies that requires...*), the verb would be singular because the antecedent of "that" would be "the only one". - **Proposition IV is INCORRECT.** When subjects of mixed person or number are joined by correlative conjunctions like "either... nor" or "neither... nor", standard prescriptive grammar does not default to a plural form; rather, it strictly adheres to the **Rule of Proximity** (or proximity agreement), where the verb agrees with the subject closest to it (e.g., *Neither the lead investigator nor the assistants were present...*). Claiming that it overrides proximity and defaults to plural is grammatically false.