Step 1: Rather than naming the rule first, rewrite the sentence with the subject simplified to test subject-verb agreement directly: "...the employees was aware of..." This immediately sounds wrong to the ear, confirming an error exists near the verb.
Step 2: Recall the applicable grammar principle: in sentences joined by "either...or" or "neither...nor", the verb does not agree with the first-named noun; it agrees with whichever noun is grammatically closer to the verb.
Step 3: Here, the two nouns are "the manager" (singular) and "the employees" (plural), and the noun closer to the verb "was" is "the employees", which is plural. A plural subject needs a plural verb, "were", not "was".
Step 4: Confirm by correcting the sentence: "Neither the manager nor the employees were aware of the changes that had been implemented overnight" reads correctly, and the only word that had to change was "was" to "were", which sits in segment (B).
Step 5: Segments (A), (C), and (D) contain no agreement, tense, or structural faults on their own.
Correct option: (1) Part B.