What is the time complexity of the following code fragment in terms of the variable n$?$

int sum $=0$;

for $($int i $=0$; i $<$ n; $++$i$)$

for $($int j $=0$; j $<$ n; $++$j$)$

sum $=$ sum $+$ i $+$ j;

Group of answer choices

O$($n$)$

O$($nlogn$)$

O$($n$2)$

O$(1)$