Step 1: Understanding the Question:
This question tests subject-verb agreement when multiple subjects are connected using parenthetical or additive conjunctions.
Step 2: Key Formulas and approach:
The sentence contains two subjects: "The Principal" (singular) and "teachers" (plural), connected by the phrase "as well as".
The grammatical rule states: when two nouns are connected by "as well as", "along with", "together with", "with", "accompanied by", or "in addition to", the verb must agree with the *first* noun.
Step 3: Detailed Explanation:
$\bullet$ The first noun in Part (A) is "The Principal", which is singular.
$\bullet$ The second noun is "teachers", which is plural.
$\bullet$ According to the rule, the verb must agree with "The Principal" (singular), requiring a singular verb.
$\bullet$ In Part (B), the verb is "were", which is plural and therefore incorrect. It must be replaced by "was".
$\bullet$ Part (C) has no grammatical issues.
Step 4: Final Answer:
The error is in Part (B), meaning Option (B) is the correct choice.