Which of the following is a declarative sentence?
-> a. john asked mary to go on a date.
b. tell me about john and mary.
c. where is john taking mary on their date?
d. john and mary are dating!

took the test its not d

The right answer is the A, since the others are, respectively, imperative (B, since it is indicating a request or an order), interrogative (C, since it is posing a question) and exclamative (D, since it is making an exclamation) sentences. A declarative sentence simply states a fact or makes a statement, as in your example: it is being said that John asked Mary to go on a date. To declare comes from the Latin declarare, which can be translated as to make known, to announce or to reveal.  

the flighty purpose never is o'ertook / unless the deed go with it"