Step 1: Understanding the Question:
We are tasked with identifying the grammatical mistake in a sentence that uses the correlative conjunction "neither... nor".
Step 2: Detailed Explanation:
According to English grammar rules, when subjects are joined by "either... or" or "neither... nor", the verb must agree in number with the subject closest to it. This principle is called the "rule of proximity."
In this sentence, the two subjects are:
1. "the manager" (Singular)
2. "the employees" (Plural)
Since "the employees" is the subject immediately preceding the verb, and it is plural, the verb must also be plural.
Thus, the singular verb "was" is incorrect and should be replaced with the plural verb "were".
The corrected sentence reads: "Neither the manager nor the employees were aware of the new policy."
The grammatical error is located in the segment "was aware".
Step 3: Final Answer:
The correct choice is (C).