C.
This is basically the definition of percent composition which is how much mass a element contributes to a specific molecule.
Sodium oxide + Water= Sodium hydroxide
A=mass number (protons + neutrons)
Z=atomic number (protons)
N=number of neutrons.
A=Z+N
Data:
A=24
N=13
Therefore:
24=Z+13
Z=24-13=11
Answer: The atomic number of this atom is 11. (The element is sodium)