Ok Lithium-6 has an atomic mass number 6 and 3 as told above.
Atomic number means the number of protons ( it also tell us number of electrons only in neutral elements ) of an element.
Atomic mass number means the number of protons and neutrons of an element.
Therefore if we take away the atomic number from the mass number we will get the number of neutrons in this element.
Therefore 6 -3 = 3 neutrons i.e The answer is B.
Hope this helps :).