What is cause of quantization of charge?
When we rub two objects only an integral number of electrons or charges can be transferred is this the reason?
Why electron has the smallest possible charge?
Why quarks cannot exist independently?
1) Technical reason is that we must have no gauge quantum anomalies. Underlying reason may be hidden in grand unification theories etc, and this leads us to conditions on charges of elementary particles.
2) Yes, since electron is elementary particle, and by the definition can't be divided.
3) Since all bodies are combined from "elementary" particles - electron and nucleons, then the summary charge of body is just the sum of elememtary charges. Also, electron has the charge which is equal to proton charge. This heuristically explains the fact.
4) The reason is strong coupling regime of low energy QCD and confinement. Formally QCD leads to the statement that interaction potential between two quarks grows linearly with distance.