Explanation:

The First Generation of computers (approximately 1940s-1950s) was primarily characterized by the use of Vacuum tubes ($ \text{c} $) as their main electronic components. These large, power-hungry, and heat-generating devices were used for circuitry and magnetic drums for memory. Option (a) Transistor was the defining component of the Second Generation. Option (d) IC (Integrated Circuit) characterized the Third Generation. Option (b) VLSI (Very Large Scale Integration) chips were the hallmark of the Fourth Generation. Therefore, Vacuum tube is the correct answer for the main electronic part of first-generation computers.