ANS:D - clearance between two mating surfaces.

Vernier calipers can be used to measure the thickness of parts, depth of holes, and clearance between two mating surfaces. However, they cannot be directly used to measure the internal diameter (I.D.) and external diameter (O.D.) of a shaft. For measuring the I.D. and O.D. of a shaft, a different type of instrument such as a micrometer or bore gauge would be more appropriate. Micrometers are designed specifically for measuring diameters with high precision, while bore gauges are used for measuring the diameter of holes. So, the correct answer is: Vernier calipers can not be used to measure the I.D. & O.D. of the shaft.