Why do a and t (and g and
c. pair in a dna double helix?

A and T pair with each other because the adenosine molecule structure has one proton acceptor, a nitrogen, and one proton donor, a hydrogen bonded to a nitrogen; the thymine molecule has one proton acceptor, an amide functional group, and a proton donor, a hydrogen bonded to a nitrogen. The proton donors and acceptors on the adenosine and thymine need to form a covalent bond with each other. 
An adenosine or a thymine cannot bond with a cytosine or a guanine because cytosine and guanine because cytosine and guanine has two proton acceptors and one proton donor, and two proton donors and one proton acceptor, respectively.

Not sure if you have any answer choices to pick. however i'd say its the intertidal zone