The correct answer is (b) pentode
The explanation: The pentode consists of an evacuated glass envelope containing five electrodes in this order: a cathode heated by a filament, a control grid, a screen grid, a suppressor grid, and a plate (anode).
Similar to a pentode valve, with which the junction FET greatly resembles in its operational characteristics, the simplest way of calculating stage gain is by the relationship –
A = gfs . RL