Answer :
The quantum number n, also called 'principal quantum number' refers to the energy level of the electron. It means that we can determine it by the integer value of the expression, like this:
[tex]\begin{gathered} 3d\text{ }\rightarrow\text{ The principal quantum number \lparen n\rparen is 3.} \\ \text{ }\rightarrow The\text{ azimutal quantum number \lparen l\rparen is d. } \end{gathered}[/tex]So, the answer is A. +3