5 is a constant. (it has no variables) x + 2 is not a constant. It does, however, have two terms, x and 2. (terms are seperated by addition/subtraction)
This rules out a and b.
As for c... 5(x+2) implies <em>multiplication</em>, which would be a product, not a sum. d. The product of a constant factor 5 and a 2-term factor x+2