Fifth Generation Computers were developed in the 1980s and marked a significant advancement in computer technology. These computers utilized several electronic components, but the correct answer is option 'D' - ULSI (Ultra Large Scale Integration).

ULSI stands for Ultra Large Scale Integration, which refers to the integration of a large number of electronic components onto a single chip. It represents a higher level of integration compared to VLSI (Very Large Scale Integration). ULSI technology allowed for the creation of complex and powerful computer systems with high processing capabilities.

Below, we will discuss the other options as well to provide a comprehensive understanding of the electronic components used in Fifth Generation Computers.

a) Transistors:
- Transistors are electronic components that control the flow of electric current in a circuit. They act as switches or amplifiers.
- Transistors were widely used in computers since the invention of the first transistor in 1947.
- However, transistors alone were not sufficient for the development of Fifth Generation Computers.

b) Integrated Circuits:
- Integrated Circuits (ICs) are miniaturized electronic circuits that are created by integrating multiple electronic components onto a single semiconductor material.
- ICs played a crucial role in the development of computers, especially in the Fourth Generation, where they replaced discrete components.
- However, Fifth Generation Computers went beyond the capabilities of traditional ICs.

c) VLSI (Very Large Scale Integration):
- VLSI refers to the integration of a large number of electronic components onto a single chip.
- It allowed for the creation of more complex and powerful computer systems compared to previous generations.
- VLSI technology was indeed used in Fifth Generation Computers, but it was not the ultimate level of integration achieved.

d) ULSI (Ultra Large Scale Integration):
- ULSI represents a higher level of integration compared to VLSI.
- It involves integrating an extremely large number of electronic components onto a single chip, providing even greater processing power and capabilities.
- ULSI technology was a key component of Fifth Generation Computers, enabling the development of advanced systems capable of artificial intelligence and parallel processing.

In conclusion, while transistors, integrated circuits, and VLSI technology were all important in the development of computers, the correct answer for the electronic components used in Fifth Generation Computers is ULSI.