The correct answer is C. Went.

1. Went: This option is correct because "went" is the past simple form of the verb "go." The phrase "It is about time" is followed by the past simple tense to express that something should have happened already or that it is overdue.

2. Go: This option is incorrect because it is the base form of the verb, which does not fit with the phrase "It is about time." The correct tense after this phrase is the past simple.

3. Have gone: This option is incorrect because "have gone" is the present perfect tense, which is not suitable for the phrase "It is about time." The past simple tense is used instead.

4. Are going: This option is incorrect because "are going" is the present continuous tense, which does not fit with the phrase "It is about time." The past simple tense is required here.

Summary:

- C. Went is the correct choice because it correctly follows the phrase "It is about time," which is used with the past simple tense to indicate that something should have happened already.