The correct answer is D. “We often have dinner at six.”
The other options are incorrect because they do not agree with the subject of the sentence, which is “we.” The verb “have” is conjugated in the present tense, which is the correct tense to use for a sentence that describes a habitual action. The verbs “having” and “has” are in the present participle and present tense, respectively, which are not the correct tenses to use for a sentence that describes a habitual action. The verb “are having” is in the present progressive tense, which is used to describe an action that is happening at the moment of speaking.
In this sentence, the speaker is describing a habitual action, so the verb “have” is the correct choice. The speaker is saying that they often have dinner at six o’clock, so the verb “have” is in the present tense.