For which pair of function is (g*f)(a)=|a|-21. f(a)=a^2-4 and g(a)= sqaureroot a2. f(a)= 1/2a-1 and g(a)=2a-23. f(a)=5+a^2 and g(a) = sqaureroot (a-5)-24.f(a) =3 -3a and g(a)=4a-5

None of the pairs will deliver (g×f)(a). If you intend (g∘f)(a), then ...

... selection 3 is appropriate.