$FFFF = 1111\:1111\: 1111\: 1111$
Look at the LSB we have $1$,so the number we will get is definitely negative.
$2$'s complement = $1$'s complement + $1$
$0000\: 0000\: 0000\: 0000\: + 1 \: =\: 1$.
But due to LSB=$1$, the answer is $-1$.
Here asked magnitude (absolute value) , so answer is $1$.