The correct option is
A Don't you
The correct answer is Option A.In the given question we are to choose the correct question tag that completes the sentence.
A question tag is a grammatical structure in which a declarative statement or an imperative is turned into the interrogative. For e.g., in the sentence "You are John, aren't you?", the statement "You're John" is turned into a question tag "aren't you". It is used to confirm the statement or encourage a reply.
In this sentence, the speaker is expecting a reply from the listener.
There are certain rules to be followed while forming question tags.
A positive statement is followed by a negative question tag and vice versa.
In the given sentence, the question tag will contain a pronoun that is in the second person because the speaker expects a reply from the person spoken to. The verb 'do' is implied in the sentence and in the statement 'I think'. The statement is positive thus the question tag will be negative. So the question tag will be the negative of 'I do think' (the first person pronoun will change to second person pronoun as explained above). Hence the correct answer is Option A- 'DON'T YOU'.
Therefore Option A is the correct answer.
Option B - DO YOU is positive and therefore is not the correct answer.
Options C and D are the question tags that can be used when the verb is DID. In the given sentence the verb is DO. Hence both the options are incorrect.
Thus Option A is the correct answer.