What is the big$-$O characterisation in terms of n$,$ of the running time of the following code segment?

void fn$($int A$[],$ int n$)\{$

if $($n $==0)\{$

return;

$\}$

int sum $=0$;

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

sum $+=$ A$[$i$]$;

$\}$

fn$($A$,$ n $/2)$;

printf$("\%$d

$",$ sum$)$;

$\}$

$($a$)$

O$($log n$)$

$($b$)$

O$($n$)$

$($c$)$

O$($n log n$)$

$($d$)$

O$($n$^2)$

$($e$)$

O$(2^$n$)$