Final answer:
The diameter of a needle is expressed in terms of its gauge, which refers to its thickness or width.
Step-by-step explanation:
The diameter of a needle is expressed in terms of its gauge. The gauge of a needle refers to its thickness or width. The higher the gauge number, the thinner the needle.
For example, a 22-gauge needle is thicker than a 25-gauge needle. Needles with different gauges are used for different purposes. Thicker gauge needles are typically used for drawing blood or injecting fluids, while thinner gauge needles are used for more delicate procedures like insulin injections.
So, the correct answer is option c. gauge.