Children generally _____________ with others of the same size and age.

The correct answer is "play." In this sentence, "children" serves as the subject, and it is plural. When using a plural subject, the verb must also be in a form that agrees with that plurality. In simple present tense, the base form of the verb "to play" is used with plural nouns. Thus, "children play" is the appropriate form to convey the idea that children typically engage in play with peers who are of similar size and age.

The other options do not fit grammatically for this context: "plays" is the third-person singular form and would be appropriate only if the subject were singular, such as "a child." "Played" is the past tense form, which does not convey the ongoing or general nature of the action. "Playing" is a gerund and does not work in this sentence structure, which requires a simple present tense verb to express a general truth.