Question

Find the nonpermissible replacement for a in this expression 3/4a

Answer 1

Under the assumption your expression is $\frac{3}{4a}$, a cannot equal 0.  If a=0, then 4a=0 and we have division by 0, which is undefined.
If your original expression was $\frac{3}{4}a$, a could be any real number.

Answer 2

they are correct the answer is 0