The ecosystem having the highest primary productivity is-
A. Pond
B. Ocean
C. Desert
D. Forest

Hint: Ecosystem is a biological community for the interaction between an organism and its physical environment. The ecosystem has biotic components and abiotic components. The biotic components include plants, animals while abiotic components include temperature, rainfall, and environmental conditions.

Complete answer: There are various types of ecosystems are as follows- Ocean Ecosystem: The primary productivity of the ocean tends to the production of organic matter by the phytoplankton and zooplankton. Green, brown, and red algae also have contributions to the primary productivity of the ocean.
Pond Ecosystem: The primary productivity of the pond includes microorganism, phytoplankton and comes under the lentic type of the ecosystem.
Desert Ecosystem: The desert ecosystem has a low level of primary productivity. This happens due to the very low amount of rainfall and high temperature which affects desert productivity.
Forest Ecosystem: It has the highest primary productivity. Primary productivity is defined as the amount of energy stored at the producer level. Then comes secondary productivity which is the energy reserved at the secondary level. Tertiary productivity is consumer-level productivity.
Hence, the correct answer is Option D.

Note: Ecosystem can be differentiated into four types which are lentic, which means the ecosystem occurs on the still bodies like ponds, lakes, rivers. It requires good exposure to the sunlight for photosynthesis. Second is the lotic ecosystem which is similar to the lentic ecosystem but occurs on moving water bodies like springs, streams. The artificial ecosystem includes beaches that are not naturally present but are man-made. The terrestrial ecosystem includes forests, deserts, grasslands that occur on land.