The deviation from the mean daily temperature to the minimum threshold temperature of a crop is termed as:

Correct option is B

A Heat Unit, also referred to as a Growing Degree Day (GDD), quantifies the accumulated heat a crop experiences above its base or minimum threshold temperature.
· This base temperature is the point below which the crop's growth is significantly slowed or ceases.
· By tracking Heat Units, growers can estimate the crop's development rate and predict key growth stages like flowering or maturity.
Other Options Explanations:
(a) Diurnal variation: Diurnal variation refers to the difference between the highest and lowest temperatures within a 24-hour period. While important for plant growth.
(c) Temperature gradient: Temperature gradient describes the change in temperature over a specific distance.
(d) Lapse rate: The lapse rate describes the decrease in temperature with an increase in altitude.