An atom of 40 18 ar has a nucleus that contains a total of
a) 18 electrons
b) 18 protons
c) 18 neutrons
d) 18 nucleons

Atoms are composed of protons, electrons, and neutrons. so, answer D can be discarded because it is not what atoms are made of. 

40 and 18 represent the atomic mass and atomic number respectively. the atomic number presents the number of protons. so there are 18 protons. the number of neutrons can be determined by taking the atomic mass minus the atomic number. neutrons= 40-18= 22 neutrons.

so the answer is B

Answer A may be correct if we assume the atom is neutral. in a neutral atom, the number of protons is the same as the number of electrons. but the electrons are not in the nucleus but orbiting it. the question specified the nucleus.

The correct answer is b