The correct answer is: B. were
The sentence is in the past tense, so we need to use a past tense verb. The verb “to be” has three forms in the past tense: was, were, and had been. “Was” is used with singular subjects, “were” is used with plural subjects, and “had been” is used with perfect constructions. In this case, the subject is “the girls,” which is plural, so we need to use the verb “were.”
The other options are incorrect because they are not in the past tense. “Had” is used with perfect constructions, but there is no perfect construction in this sentence. “Had being” is not a correct verb form.
In conclusion, the correct answer to the question “What were the girls playing at noon yesterday?” is “were.”