11
This is a matter of knowing your definitions.
Atomic number = number of protons in an atom. All elements of the same type have exactly the same number of protons.
Atomic mass = sum of the number of protons plus the number of neutrons. Any given element may have several different isotopes that each have an unique number of neutrons.
So we have an atom of boron, that automatically tells us that it have 5 protons since every element of boron has 5 protons. It's electrically neutral, so it also has a matching number of electrons. And this particular atom also has 6 neutrons. So we add 5 plus 6 giving us 11 as the atomic mass. Looking at the 4 choices of 6, 5, 11, and 16, the only possible answer is 11.
