Hello! Your answer is B, 7.
Generally speaking, and atom usually has the same number of protons (the atomic number) and neutrons. However, there is such thing as isotopes - variants of the number of neutrons. Therefore, we cannot just immediately say there are 7 neutrons.
If an element has an atomic number of 7, this means it has 7 protons, as the proton count correlates to the atomic number. Each proton weighs 1 amu. Therefore, all the protons in the atom weigh 7 amu. Since the atomic mass of nitrogen is 14, we know that the mass of what is not protons is 7 amu.
There are two things in an atom which are not protons. Neutrons and electrons. Between these two, neutrons are the one which mainly contribute to weight - electrons weigh a minuscule amount. Each neutron weighs 1 amu. If we have 7 amu left over, and each neutron weighs 1 amu, there must be 7 neutrons.
Hope this helps!
