Answer:
C). Swim.
Step-by-step explanation:
As per the question, the present tense of the verb that would complete the given sentence would be 'swim' as it exemplifies a regular action and follows the tense(present) and subject-verb agreement(plural verb 'swim' with the plural subject 'they').
Option A is incorrect as it offers a past form of the verb while option B falsely proposes 'am' as a helping verb which would be inappropriate with the pronoun 'they'. Option D is incorrect as the sentence exemplifies a regular action that could not be stated in the present perfect tense. Therefore, option C 'swim'(present indefinite tense) would best complete and accomplish the meaning of the sentence.