Which of the following particles cannot be found inside other particles?

Leptons are fundamental particles that do not have any substructure; they are not made up of smaller particles. This characteristic distinguishes them from quarks, baryons, and mesons. Quarks combine to form baryons (such as protons and neutrons) and mesons, which are made of quark-antiquark pairs.

In contrast, baryons—which include particles like protons and neutrons—are composites made of three quarks. Mesons consist of a quark and an antiquark. Since leptons stand alone and do not combine to form more complex structures, they cannot be found inside other particles. This distinct nature of leptons clarifies why the answer is correct.

Understanding the basic composition of these particle types is crucial, as it defines their role in the Standard Model of particle physics and how they interact with one another.