The correct answer is (c), temperate grasslands.
Temperate grasslands have the lowest mean net primary productivity because they have a short growing season and a high level of herbivory. The short growing season means that plants have less time to photosynthesize and produce biomass. The high level of herbivory means that a lot of the biomass that is produced is eaten by animals, rather than being stored as plant matter.
Temperate forests have a longer growing season and a lower level of herbivory, so they have a higher mean net primary productivity. Tropical forests have a very long growing season and a low level of herbivory, so they have the highest mean net primary productivity. Tropical savannas have a long growing season and a moderate level of herbivory, so they have a mean net primary productivity that is intermediate between temperate forests and tropical forests.
Net primary productivity is the amount of energy that is stored in plant matter as biomass per unit area per unit time. It is a measure of the productivity of an ecosystem.