The only information we are given is the mass number, so we have to work our answer from the options.

Using the elimination method, options B and C are incorrect because when you add the numbers together, they give 47 and 46 respectively which is more than 31.

We are left with options A and D. However, the correct answer will be option A, since there are 15 electrons which invariably means, we also have 15 number of protons. 

⇒ The element has atomic number of 15 (Phosphorus)