When considering conventional current versus electron current flow:

ANS:C - conventional current flow came first

Conventional Flow is considered at first before they discovered that the electrons are actually the ones moving from the negative to the positive side. Technically it is still the same and is useable for any kinds of circuits, it just depends on the perspective of the person looking at the circuit if he/she wants to use a conventional flow or current flow. But according to history, the first thought of people was the conventional flow so it came first that's why C is correct. Also, protons cannot flow since they are located in the nucleus of an atom. Only electrons can be dislodged and flow in the form of a current.