The correct option is C belong
In order to select the correct form of the verb to complete the sentence, let's understand the subject-verb agreement. Subject and verb should agree with each other, i.e., if the subject is singular, the verb should also be in its singular form; and if the subject is plural, the verb should also be in its plural form.
NOTE: 'You' is considered to be a plural pronoun.
Option A - The word 'do' in the question explains us that the question is framed in present tense. As explained above, the given subject in the question 'you' is a plural subject, hence we need a verb in its plural form (without 's/es'). 'Belongs' is a singular subject, hence, we can't use it here. Thus option A is incorrect.
Option B - 'Belonging' is a verb in its participle form. However, the word 'still' explains that the question is in simple present tense, hence, we can't use 'belonging' here. Thus option B is incorrect.
Option D - As the question is in framed in simple present tense, we can't use a verb in past tense. 'Belonged' is the past form of the verb 'belong'. Hence, option D is incorrect.
Option C - As you is a plural subject, we need a verb that is in its plural form. A plural verb in present doesn't have 's/es'. Hence, we can use 'belong' here. 'Do you still belong to the football club?' is a correct formation. Thus option C is the correct answer.