Which of the following exhibits least productivity?

The correct option is A Open oceans
Productivity denotes the rate of production of biomass in the ecosystem. Although in the ocean, there are lots of nutrients but the productivity is less because the light is only available in the photic zone of ocean and deeper down light does not penetrate and hence the ocean has lesser productivity.

Maximum productivity can be seen in coral reefs followed by estuaries, swamps and marshes, tropical rainforest and temperate rainforests.

Deserts, tundra and the open ocean are the least productive ecosystems typically having less than 0.5 x 10³ kcal.