An atom is electrically neutral. So, in an atom
A) Number of protons = Number of neutrons, in the nucleus.
B) Number of electrons = Number of neutrons, in the nucleus
C) Number of particles = Number of Y particles
D) Number of electrons = Number of protons in the nucleus.

Hint:An atom is made up of three particles namely electron, proton and neutron. Electrons are negatively charged particles, protons are positively charged particles and neutrons are neutral. There are many subatomic particles that are in the atoms some of them are: Hadrons, Bosons, Muons, quarks, etc.

Complete step by step solution:
For an atom to be balanced or to be electrically neutral there has to be a balance between the negatively charged particles (electrons) and positively charged particles (protons). Suppose the number of protons is more than the number of electrons. The atom will get positively charged, which will not remain electrically neutral, an atom cannot remain in an electrically state for a long time. Similarly, if there are more electrons in the atom. The atom will get negatively charged and cannot stay in the excited state for a long time. The atom also plays a role in keeping the atom electrically neutral although the neutron is electrically neutral but it does a job of maintaining the atom electrically balance, suppose there are 10 protons and 8 electrons in the atom. The presence of neutrons will compensate for the 2 extra protons and hence the atom will remain electrically neutral. Neutron can only adjust a slight variation in the number of protons and electrons, if in an atom the number of protons is 10 and the number of electrons is 4 then the neutron wouldn’t be able to adjust the extra number of protons and the atom will be positively charged.

Option “D” is correct. An atom is electrically neutral. So, in an atom the Number of electrons = Number of protons in the nucleus.

Note: The structure of the atom is such that the neutron and proton remain in the nucleus which contributes to the major part of the atom weight. Electrons revolve around the nucleus just like our solar system planets revolve around the sun in an elliptical orbit similarly, in atom electrons revolve around the nucleus in a particular orbit known as shell.