Answer:
The correct option is: c. 15
Step-by-step explanation:
Phosphorous is a chemical element which belongs to the group 15 of the periodic table and has atomic number 15. It is a highly reactive non-metal of the p-group. 
Since, atomic number of an atom is the number of electrons and number of protons for neutral atoms. 
So, the number of protons = number of electrons = 15 
The atomic mass is obtained by adding the number of neutrons and the protons. 
So, number of neutrons + number of protons = 30 
So, number of neutrons + 15 = 30 
Therefore, the number of neutrons in ³⁰P = 15