Select the Right Expression for the Following Sum and Prove $P _ { n } : 3 + 6 + 9 \ldots 3 n$

Multiple Choice

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

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