Answer
$ V = 20 V$
Work Step by Step
We are told that the current is 10 Amps and that the Resistance is $2 \Omega$. Thus, we use Ohm's Law to obtain:
$V = IR = (10A)(2 \Omega) = \fbox{20V}$
Thus, it follows that the missing element is a $\underline{20V \ voltage \ source.}$
