Answer:
A quark
Explanation:
Atoms are the fundamental particle of the matter. An atom consist of a proton, a neutron and electron revolves around the nucleus. The neutrons and protons are composed of quarks. It is a type of elementary particle.
The quarks are of six types i.e. up, down, charm, strange, top and bottom. There are also antiparticles of quarks known as anti quarks.
So, a question mark in the diagram is "a quark". Hence, this is the required solution.
