The correct answer is option 'A' - MIPS (Million Instructions Per Second).

Explanation:
Mainframe computers are high-performance computing systems used by large organizations for processing massive amounts of data and running critical applications. The speed of a mainframe computer is measured in MIPS, which stands for Million Instructions Per Second. Let's break down the answer and understand it in detail.

1. Mainframe Computers:
Mainframe computers are powerful machines designed to handle complex tasks and process extensive amounts of data. They are typically used by large corporations, government agencies, and financial institutions where high processing power, reliability, and security are essential.

2. Speed of Mainframe Computers:
The speed of a computer refers to how quickly it can execute instructions and perform calculations. In the case of mainframe computers, their speed is measured in MIPS, which stands for Million Instructions Per Second.

3. MIPS:
MIPS is a unit of measurement used to quantify the performance and processing speed of a computer system. It represents the number of instructions a computer can execute in one second. A higher MIPS value indicates a faster and more powerful computer.

4. Importance of Speed Measurement:
The speed measurement of mainframe computers is crucial for determining their efficiency and capability to handle complex tasks. Organizations rely on mainframes to process large volumes of data and perform critical operations swiftly. Therefore, the speed measurement helps in evaluating the overall performance and suitability of a mainframe computer for specific applications.

In conclusion, the unit of measurement used to determine the speed of a mainframe computer is MIPS (Million Instructions Per Second). This measurement quantifies the computer's processing capabilities and plays a vital role in assessing its performance and efficiency.