Option 1: Mercury thermometer is not suitable for measuring temperatures as high as 1250°C. This is because mercury tends to evaporate at high temperatures, which can affect the accuracy of the measurement. Alert - correct answer should be .

Option 2: Constant volume gas thermometer is not the preferred choice for measuring temperatures of 1250°C. Constant volume gas thermometers are typically used for lower temperature measurements, as they rely on the relationship between pressure and temperature. At higher temperatures, the pressure inside the thermometer can become difficult to measure accurately.

Option 3: Optical pyrometer is the preferred thermometer for measuring temperatures around 1250°C. Optical pyrometers work by measuring the intensity of the light emitted by the object being measured. This light intensity is then correlated to the temperature of the object. Optical pyrometers are commonly used in industrial settings for measuring high temperatures. They are non-contact thermometers and can provide accurate measurements at high temperatures.

Option 4: Platinum resistance thermometer is not ideal for measuring temperatures as high as 1250°C. Platinum resistance thermometers have a limited temperature range and may not be able to provide accurate measurements at such high temperatures.