If $f$ is a real values function satisfying $f(x+y)=f(x)\;f(y)$ and $f(1)=a$ where $x,y\in R$, then $f(x)$ =

Question

(A) $x^a$

(B) $a^{2x}$

(C) $a^{x/2}$

(D) $a^x$