Which atomic particle of these 4 is the smallest?

Did you got it? If no, don't be upset as in school we were taught that matter is made of protons and neutrons. But now scientists tell us that these particles are composed of even smaller parts known as quarks. There are 6 types of quarks though physicists prefer to call them six "flavors" of quarks. Each of them has its corresponding antiquark. The theoretical issues concerning the particle were elaborated in the mid-60s but the first quark was discovered only in the latter half of the 20^th century.

As for the photon, this particle is not an atomic but an elementary one and it has a dual nature.