(a) Internal resistance is the resistance which is present within the cell that resists the current flow inside the cell when connected to an external circuit with some external resistance. Thus, it causes a voltage drop when current flows through it.

It is the resistance provided by the electrolyte and electrodes which is present in a cell. So, internal resistance is offered by the electrodes and electrolyte which oppose the current flow inside the cell.
(b) (i) V vs. R graph (ii) V vs. I graph