Find the union of the following sets $A=\{x : x$ is a natural number and multiple of $3 \},B=\{x : x$ is a natural number less than $6\}$

Question

$\begin{array}{1 1}(A)\;\{1,2,3,4,5,6\}\\(B)\;\{3,6,9\}\\(C)\;\{3,9,81...\}\\(D)\;\{1,2,3,4,5,6,12,15.....\}\end{array}$