Concept:The accurate measurement of an internal dimension, such as the inner diameter of a small test tube, requires an instrument with jaws designed to fit inside the opening.
Explanation:A micrometer screw gauge is best for measuring the external diameter of small objects, not internal cavities.
A pair of dividers only transfers distances and cannot give a direct reading of the inner diameter.
A metre rule has too low a resolution and cannot easily measure the inside of a small tube.
A pair of vernier callipers has both external and internal jaws.
The internal jaws are inserted into the test tube and adjusted until they touch the inner walls.
The vernier scale then gives a precise reading of the inner diameter.
Because the test tube is small, the vernier calliper offers the necessary accuracy and accessibility.
Answer:D. Pair of vernier calliper