Which of the following is the atomic number of a divalent cation of an element containing 10 electrons?       

The atomic number of a divalent cation of an element containing 10 electrons. is 12. An atomic number of an atom is equal to the number of electrons in a neutral atom. 

Cations are formed by the loss of electrons. The number of electrons lost is equal to the charge present over the cation. 

Now since it is a divalent cation it has lost two electrons or we can say it has a positive charge of +2 on it and hence has two electrons less than the neutral atom so the number of electrons in total is 12 hence the atomic number is 12.