Tough question. I'm a little rusty, since I took AP Chem a while back. First off, Group 7A refers to the second-to-last column (going from left to right) on the periodic table. This group is also called the "Halogens". Furthermore, the halogens all form binary compounds with hydrogen known as the hydrogen halides (HF, HCl, HBr, HI, and HAt), a series of particularly strong acids (pulled from Wikipedia). Now, your question: which is the strongest? If I recall correctly, the acid strength of each of these halides INCREASES as you go down the group. That being said, the answer would either be HI or HAt. I would choose HI, since HAt is very uncommon since Astatine is rare, unstable, and not found in very substantial quantities. So, your answer would be hydrogen iodide.

Doesn't this question belong in "Homework Help" instead. Bump!