Which of the following quantum number combinations is not allowed in an atom?

A. n = 2, l = 1, mr = - 1

B. n = 1, l = 1, m = 0

C. n = 8, l = 5, m = - 4

D. n = 6, l = 3, m = 2

The correct answer in this question is option B. The combinations n = 1, l = 1, m = 0 is not allowed in an atom. One of the rules in quantum number combinations is that the angular quantum number (l) can be any integer from 0 to n-1. In option B, the principal quantum number (n) is equal to the angular quantum number.