Divide f(x) over g(x). To simplify, factor everything as much as possible and cancel out any common terms. In this case "4x+3" is a common term that shows up in the numerator and denominator.
(f/g)(x) = [ f(x) ]/[ g(x) ]
(f/g)(x) = (16x + 12)/(4x + 3)
(f/g)(x) = (4(4x+3))/(4x+3)
(f/g)(x) = 4
So the answer is choice C