Early computers took up entire rooms. which of these dramatically reduced the size of computers?
microchips
transistors
compact discs
digital-video discs

Transistors were the component that dramatically and primarily reduced the size of computers.

Explanation:

In the late 1950's, computers could take up entire rooms, and relied on vaccuum tubes, which often became too heated and were not quite reliable. Computers often burned out. The first major discovery that allowed a significant size reduction and reliability increase in computers, was the implemmentation of individual transistors and the use of magnetic core memory. These were referred to as the Second Generation Computers, popular throught late 1950's and 1960's.