Concept:Diameter is the straight-line distance across the centre of a circle, so it is a type of length.
Explanation:To measure any length, we use an instrument calibrated in units of length.
A metre-rule is designed to measure length directly.
A circle’s diameter is a straight length, so a metre-rule can be used to measure it.
A thermometer measures temperature, not length.
An ammeter measures electric current, not length.
A string is better used for curved lengths; it does not measure the diameter directly.
Answer:A. A metre-rule