Answer:
B. Ca
Explanation:
Let's look at the electron configurations of all four elements (I am going to write it in noble gas configuration to make it simpler):
Mg electron configuration: [Ne]3s2
Ca electron configuration: [Ar]4s2
Ar electron configuration: [Ar]
K electron configuration: [Ar] 4s1
We notice that Ca has two electrons in the 4s sublevel, which satisfies what the question is asking for.
The answer is thus B. Ca.
