Element A has 15 protons. How many neutrons are there in its isotope with mass number 33?

A) 15

B) 18

C) 33

D) 48

I think the correct answer from the choices listed above is option B. The mass number of every isotope of an element is equal or constant. It is the number of neutrons that id different with every isotope. For this case, the number of neutrons should be 18 since the number of neutrons and the number of protons should sum up to the mass number.

The number of neutrons that are present in this isotope with a mass number of 33 would be B. 18.