Which generation of computer used transistor?

The use of transistors marks the second generation of computers (roughly 1959–1965).

First generation computers used vacuum tubes; they were large and generated a lot of heat.

Second generation computers replaced vacuum tubes with transistors, making them smaller, faster, more reliable, and more energy-efficient.

Third generation computers used integrated circuits (ICs), and fourth generation computers use microprocessors.

Therefore, transistors were used in the second generation of computers.