Let $f$ be a non-zero real valued continuous function satisfying $f(x+y)=f(x).f(y)$ for all $x,y \in R.$ If $f(2)=9,$ then $f(6)=$

Question

$$(a)\;3^2 \quad (b)\;3^6 \quad (c)\;3^4 \quad (d)\;3^3$$