What is the time complexity of the following algorithm of computing the sum of the first n nonzero natural numbers?

input n

sum $=$ n$*($n$+1)/2$

output sum

$[$ A $]$ O$($n$)$

$[$ B $]$ O$($n$2)$

$[$ C $]$ O$($c$),$ where c is a constant.

$[$ D $]$ O$($n$3)$

$[$ E $]$ O$($n$*($n$+1)/2)$