Concept:A vernier calliper is designed to measure both internal and external dimensions accurately.
Explanation:The question asks for the instrument used to measure the external parts of an object accurately.
A meter rule gives limited accuracy.
A micrometer screw gauge is highly accurate but mainly for small external measurements like wire diameter.
A vernier calliper can measure external diameters, internal diameters, and depths with good accuracy.
It has jaws for external measurements and smaller jaws for internal measurements.
Therefore, it is the most suitable instrument among the options for measuring external parts accurately.
Answer:B. Vernier calliper