Question

.

Answer 1

Why D?

Answer 2

Ka= [H+][A-] / [HA]
an ideal buffer is a 50% : 50% mix of [A-] & [HA]
in which case the buffer is in the middle of its buffer range
&
since at 1/2 way in its range, [A-] = [HA]
then
K = [H+]

Answer 3

pH=-logKa
4.5=-log Ka
-4.5=logKa
10^-4.5=Ka

Answer 4

It should be acetic acid you can check
pH=-log(1.85x10^-5)
It should be 4.5 you can do the same to the others to see what their pH would be