Answer
$3$ $MeV$
Work Step by Step
Total energy of the proton= kinetic energy of the proton+potential energy of the proton
$\implies E=K+U$
If the proton reflects from the barrier, the value of $U=0 $ on the reflected side of the barrier because the the height of potential barrier is zero in this region.
Therefore, $K=E=3$ $MeV$
$\therefore$ If the proton reflects from the barrier, the kinetic energy of the proton remains $3$ $MeV$
