Steel Bulb Thermometer and its Liquid
A steel bulb thermometer is a type of thermometer that uses a steel bulb filled with a liquid to measure temperature. The liquid inside the bulb expands or contracts depending on the temperature, causing the level of the liquid to rise or fall in the thermometer's capillary tube. By measuring the level of the liquid, the temperature can be determined.

Measurement Range Requirement
In this question, the requirement is to measure temperatures up to 6000C. Therefore, the liquid used in the steel bulb thermometer should be able to accurately measure temperatures within this range.

Options for the Liquid
Let's analyze each option to determine which one is suitable for measuring temperatures up to 6000C.

a) Mercury: Mercury is commonly used in steel bulb thermometers because it has a wide temperature range and high thermal expansion coefficient. It can measure temperatures from -38C to 3560C. Therefore, mercury is a suitable option for measuring temperatures up to 6000C.

b) Ether: Ether is a volatile and highly flammable liquid. It has a lower boiling point compared to mercury, making it unsuitable for measuring temperatures up to 6000C.

c) Water: Water boils at 100C and freezes at 0C, which means it has a limited temperature range. It cannot accurately measure temperatures up to 6000C.

d) None of the mentioned: This option is incorrect because mercury can indeed measure temperatures up to 6000C.

Conclusion
Based on the analysis, the correct option is a) Mercury. It is the most suitable liquid for measuring temperatures up to 6000C in a steel bulb thermometer due to its wide temperature range and high thermal expansion coefficient.