The correct conjugation of the given sentence, assuming that it is in the present continuous tense is "Los chicos están jugando en la arena."
This refers to the variation of verb forms in order to indicate gender, time, number, etc in a given sentence.
Hence, we can see that from the base word which is "jugar", we need to show the action of the boys and we assume this is in the present continuous form, so the answer is "Los chicos están jugando en la arena."
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