Final answer:
A micrometer (μm) is a unit used to measure length, specifically one millionth of a meter. In the provided context, the diameter of a human hair is given as 50 μm, indicating this is a measure of length.
Step-by-step explanation:
The symbol μm stands for micrometer, which is a unit of measurement for length. A micrometer is equal to one millionth of a meter (10-6 meter). In the context of the question, a human hair is approximately 50 μm in diameter, which refers to the thickness or width of the hair. This is a very small measurement, demonstrating the precision needed when dealing with objects of such tiny dimensions, similar to those observed under a microscope or in the field of micro-engineering.
It is clear that μm is used to measure linear dimensions, such as diameter, which gives us the answer to the student's question: 'b) Length'.