Out of the following temperature measuring instruments, the measurement accuracy will be minimum for the

Answer: Option B

Optical pyrometers measure temperature based on the visible radiation emitted by hot objects and are typically used for very high temperatures where contact methods are impractical.

However, they have minimum measurement accuracy among the options because they depend on the emissivity of the object's surface, which can vary widely with material, surface finish, and oxidation, introducing significant errors.

Mercury in glass thermometers (Option A) and alcohol filled thermometers (Option D) offer good accuracy for lower temperature ranges with direct contact measurement.

Iron-constantan thermocouples (Option C) provide reliable and accurate measurements over a wide range of temperatures with reasonable precision.

Therefore, due to their reliance on visual estimation and sensitivity to surface emissivity, optical pyrometers have the lowest measurement accuracy among the instruments listed.