The correct answer is: c) The number of crops grown on a plot in a year.
Cropping intensity is the number of times a piece of land is cultivated in a year. It is calculated by dividing the total area cultivated by the net area sown. A high cropping intensity indicates that the land is being used efficiently.
Option a) is incorrect because it refers to the number of different crops grown, not the number of times a piece of land is cultivated.
Option b) is incorrect because it refers to the proportion of land used for cultivation, not the number of times a piece of land is cultivated.
Option d) is incorrect because it refers to the productivity per unit area, not the number of times a piece of land is cultivated.