Final answer:
To measure the temperature of the frosty in the hopper, a thermistor probe or thermocouple probe should be used as they are designed for insertion into a substance to assess its temperature accurately option 4).
Step-by-step explanation:
To check the temperature of the frosty in the hopper, one should ideally use a probe that can accurately measure the temperature of semi-solids. Out of the options provided, a thermistor probe or a thermocouple probe would be suitable for this task. Both of these probes have the ability to measure the temperature by being inserted into the substance.
An Infrared thermometer probe is more suitable for non-contact surface temperature measurements and may not provide an accurate temperature of the frosty inside the hopper. A Resistance Temperature Detector (RTD) probe is generally used for precise temperature measurements, but it's usually more costly and may not be necessary for a routine check like this.