In an electrically neutral atom of any element, there are equal numbers of
a) protons and neutrons.
b) electrons and protons.
c) electrons and neutrons.
d) atomic number and atomic mass.
eliminate

B) Electrons and protons  

Explanation:  

Each proton has a positive charge, and each electron has a negative charge. If the atom is electrically neutral, there must be the same number of electrons and protons.

A) and C) are wrong. Neutrons have no charge, so they can't counteract that of the protons or the electrons.

D) is wrong. The atomic number is the number of protons, and the mass number is the number of protons plus neutrons. The neutrons can't  counteract the charge of the protons,

the answer is b. dilution