Which of the following sentences is grammatically correct?

The sentence "She does have a snack" is grammatically correct because it adheres to the standard structure of the English language for the present tense when used with the third-person singular subject "she." In English, the auxiliary verb "does" is used with the base form of the verb "have" to create a correct affirmative statement. This construction is essential in proper subject-verb agreement, especially in the present tense, where "does" helps to clarify the subject and maintain consistency in the verb form.

In contrast, the other options do not follow this grammatical structure. For instance, "She do have a snack" incorrectly uses "do" instead of "does," which is not the correct form for a third-person singular subject. The option "She having a snack" lacks a proper verb form to indicate a complete action, as it doesn't include an auxiliary verb or the correct verb conjugation. Lastly, "She has snack" is missing the article "a" before "snack," which is necessary to denote the singular form appropriately. Hence, the sentence "She does have a snack" stands out as the only grammatically correct option.