To solve this question, we need to look for a pattern or sequence in the given series: M V E M J S U N ?.
If we observe closely, we can see that each letter in the series represents the first letter of the names of the planets in our solar system in reverse order.
M - Mercury
V - Venus
E - Earth
M - Mars
J - Jupiter
S - Saturn
U - Uranus
N - Neptune
So, the missing letter in the series is the first letter of the planet before Neptune, which is P.
Therefore, the correct answer is D) P.