Final answer:
The correct answer is 'a. was' because the subject 'One of the girls' is singular.
Step-by-step explanation:
The correct answer is option a. was.
In this sentence, the subject 'One of the girls' is singular, so a singular form of the verb 'to be' should be used. 'Was' is the correct past tense form of the verb 'to be' for a singular subject.
For example, you can say 'One of the girls was late for the class', or 'She was late for the class'.
Learn more about Subject-Verb Agreement