Question

for which pair of functions is (g*f)(a)=|a|-2?
a. f(a)=a^2-4 and g(a)= sqrt a
b. f(a)= 1/2s-1 and g(a) = 2a – 2
c. f(a)=5+a^2 and g(a)= sqrt a-5 -2
d. f(a)=3-3a and g(a)=4a-5
please helpp