Which of the following particles is a lepton?
A. Electron.
B. Proton.
C. Neutron.
D. $\pi - $meson.

Hint: A lepton is an elementary particle that has a half-integer spin which does not show strong interaction. Leptons have properties including spin, electric charge, and mass.

Complete step by step solution:
We can consider each option and then find the correct answer to the problem.
Let us consider our first option,
Electron:
Leptons have the capability to carry unit charges or they can remain neutral. Leptons in the charged state are electrons, muons, and taus each of these charged particles have a negative charge and distinct mass.

Let us consider the second option,
Proton:
The charge that lepton can have is negative and not positive. The lepton densities in protons are extremely small.

Let us consider the third option,
Neutron:
The neutrons are made of quarks that show strong forces but particles that are made up from leptons show weak forces which mean the neutron doesn’t contain leptons.
Let us consider the fourth option
$\pi - $Meson.
$\pi - $meson is the particle which is composed of elementary particle quarks and antiquarks having strong bound. $\pi - $meson is the particle that does not contain lepton as an elementary particle.

After comparing all the options it is clear that electrons have leptons as elementary particles.

Note:
There are elementary particles like quarks, anti-quarks, leptons, and antileptons. All the particles are made of these elementary particles only. The particles which are made of quarks and antiquarks are shown strong forces but the particles which are made up of other than these particles show weak forces.
Protons and neutrons are made up of quarks and antiquarks having strong bonds but electrons have elementary particles as leptons.