If n=2, and l=0, then what are the possible values of ml?

The answer is 0.

n is the principal quantum number and l is the angular momentum quantum number. The possible values of ml (magnetic quantum number) range from - l to + l. In this case, 0 is the only possible value of ml.

But, as a further example, if n=2 and l=1, the possible values of ml would be - 1, 0, and + 1.