There . . . . . many children on the playground yesterday. A. are B. were C. did D. playing

The correct answer is: B. were

The sentence is in the past tense, so the verb “to be” must be conjugated in the past tense as well. The past tense form of “to be” is “were”.

The other options are incorrect because they are not in the past tense.

• Option A: “are” is the present tense form of “to be”.
• Option C: “did” is the past tense form of the verb “to do”.
• Option D: “playing” is the present participle form of the verb “to play”.

Here is a complete sentence using the correct verb:

There were many children playing on the playground yesterday.