Right answer is (a) Climatological Factors
The best explanation: Potential Evapotranspiration (PET) involves transpiration process. As this process consists of transpiration losses through leaves of plant, and evaporation losses from surroundings of the plant and this directly dependent on availability of moisture which when is sufficiently available to meet the needs of the vegetation then it is called PET. Therefore this clearly states PET critically depends on climatological conditions.