This sentence show a habit of going to park every Sunday. The answer will be ‘Go’ because it is in simple present tense. This tense is used for things that are habitual. Few more examples -I go to school (this is something done habitually/regularly)I wake up at 7 am. (This is usual time at which I wake, so we use simple present tense) :)

Correct answer: They go to the park every Sunday.

Explanation:
In the given sentence, the correct verb tense to complete the sentence is "go".

Present Simple Tense:
The present simple tense is used to describe actions that are habitual, regular, or true in general. In this case, the sentence is describing an activity that happens every Sunday, which makes it a regular habit. Therefore, the present simple tense is the appropriate verb tense to use.

Subject-Verb Agreement:
In English grammar, the verb must agree with the subject in number and person. In this sentence, the subject "They" is a third-person plural pronoun, so the verb must be in the third-person plural form. The present simple tense for third-person plural subjects is formed by adding "-s" or "-es" to the base form of the verb. However, in this case, the base form of the verb "go" remains unchanged because it is an irregular verb. Therefore, the correct form is "They go".

Examples of Present Simple Tense:
Here are a few more examples of sentences using the present simple tense:

- She plays the piano every day.
- The sun rises in the east.
- We eat dinner at 6 o'clock.

In summary, the correct verb tense to complete the sentence "They _____ to the park every Sunday" is "go". It is in the present simple tense, which is used to describe habitual or regular actions. The subject-verb agreement requires the verb to be in the third-person plural form, which is "go" in this case.