Final answer:
Quarks were first proposed in 1963 by Murray Gell-Mann and George Zweig. The existence of various quark flavors has been confirmed through experiments, although direct observation has been challenging. Quarks are the building blocks of protons and neutrons.
Step-by-step explanation:
Quarks were first proposed independently by American physicists Murray Gell-Mann and George Zweig in 1963. Originally, three quark types or flavors were proposed with the names up (u), down (d), and strange (s). In 1974, two groups of physicists independently discovered a particle with the new quark, labeled charmed (c). A final pair of quarks, bottom (b) and top (t), were proposed and verified in 1976 and 1995, respectively. Quarks are incredibly small and have not been observed directly due to the strong force that binds them together. However, indirect evidence such as scattering experiments has confirmed their existence.