Concept:A thin wire has a very small diameter, so it needs an instrument with very high precision.
Explanation:The diameter of a thin constantan wire is usually only a fraction of a millimetre.
A micrometer screw gauge can measure small diameters accurately up to
0.01 mm.
A vernier calliper can measure only up to
0.1 mm, which is not precise enough for this purpose.
A metre rule is far too coarse, and ordinary callipers are also less precise.
So the micrometer screw gauge is the most suitable instrument for measuring the diameter of a thin constantan wire.
Answer:C. micrometer screw gauge