D, and ONLY D!
The rule with tag questions is that if the first part is positive, the tag is negative and vice versa. There are some exceptions but they are not relevant here.
In your sentence, "There are a lot of people here,.....?" the answer "aren't there?" makes sense as a correct tag because you need a negative tag (the first part, are, is positive) plus the "there is/are" structure is used in the sentence, which means that it needs to be used in the tag and NOT the personal pronoun "they".