A thermistor has

Answer: Option A

Positive temperature coefficient: This is the correct answer. A thermistor with a positive temperature coefficient (PTC) exhibits an increase in resistance with an increase in temperature. The resistance-temperature relationship is positive for PTC thermistors.
Negative temperature coefficient: This is not the correct answer. A thermistor with a negative temperature coefficient (NTC) exhibits a decrease in resistance with an increase in temperature. The resistance-temperature relationship is negative for NTC thermistors.
Zero temperature coefficient: This is not the correct answer. A zero temperature coefficient would mean that the resistance does not change with temperature, which is not a characteristic of thermistors.
Variable temperature coefficient: This is not the correct answer. Thermistors typically have a well-defined temperature coefficient, either positive or negative, and are not variable in this aspect.
Therefore, in the context of the question, a thermistor has a Positive temperature coefficient.