a) Helium (he) has the smallest atomic mass
why? because He's atomic number is the smallest of the lot and as the atomic number increases, the atomic mass increases, with the exceptions of few isotopes.

15)
a- acids taste sour NOT bitter
b- if by ph paper you mean litmus paper then no, acid turns litmus paper red
c-acids react with BASES to produce water and salt
d - yes acids do react with SOME metals to produce hydrogen gas, it doesnt work with metals like copper though..

I think the answer for the Which elements most often share electrons with carbon to form large biological molecules is
c- nitrogen, hydrogen, oxygen because these along with carbon are the top 4 most elements found in our body.. chlorine isnt even in the top ten list ( somebody correct me if i am wrong about chlorine not being in top ten)

the only halogen out of all the ones you gave is Flouride
so a) HF should be the answer