Final answer:
The correct answer is b) NSU.
Step-by-step explanation:
In official letters or messages, the abbreviation NSU stands for 'Not Suitable for Use' or 'Not Serviceable Unit'. It is used to indicate that an asset is not suitable or functional for its intended use. This could mean that the asset is damaged, malfunctioning, or otherwise not fit for its intended purpose.
For example, if an official letter mentions that a piece of equipment is NSU, it means that the equipment is not suitable for use and cannot be used until it is repaired or replaced.