Choose the grammatically correct option to complete the sentence:

The correct answer is has.

Explanation:
In English grammar, subject-verb agreement requires that singular subjects take singular verbs, while plural subjects take plural verbs.

When selecting the appropriate verb form for a sentence with a third-person singular subject (such as he, she, it, or a singular noun/uncountable noun), the singular present tense form has is used.

Let's evaluate the given options:

1. have: Used with plural subjects (and I/you/we/they), not suitable for singular third-person subjects.
2. has been being: Highly redundant and grammatically awkward in standard English usage.
3. has: The correct third-person singular present tense verb.
4. is: A present tense form of the verb to be, which does not convey possession or serve as an auxiliary in the standard perfect tense context required here.

Therefore, has is the grammatically correct option to complete the sentence.