Select the Right Expression for the Following Sum and Prove $n$

Multiple Choice

Select the right expression for the following sum and prove that your statement is true for all natural numbers $n$ .
$$P _ { \mathrm { n } } : 3 + 6 + 9 \ldots 3 n$$

A) $\quad \frac { 3 n ( n + 1 ) } { 4 }$
B) $\frac { n ( 3 n + 1 ) } { 2 }$
C) $\quad \frac { 3 n ( n + 1 ) } { 2 }$
D) $\frac { n ( 3 n + 1 ) } { 4 }$
E) none of the above