Answer:
The correct answer is triode.
Explanation:
A vacuum tube is a device that control the flow of electric current between its electrodes in the presence of vacuum. The first vacuum tube which was used have two electrodes and called diode. Then another vacuum tube was invented having a grid between its two electrodes anode and cathode. So, that grid control the flow of electric current between these two electrodes. So, this tube named as triode named as it have one grid and anode and cathode. Other tubes which have more than one grid called according to there number of electrodes totally like tetrode, pentode.
I hope it will help you!
