That's a good question. What did the explanation say? I was thinking D too because ground state Sr would be 5s^2; 5 is right and s refers to 0. 1 would be p for azimuthal.
The book is decent... I went thru destroyer but all the calculations were difficult so I am using this book. It's mostly very conceptual only a few calculations but I think I get the grasp. The bootcamp online is hard with the math portion of the gen chem.