Despite occupying about 70% of the surface area of earth, the product…

The correct answer is Option (1) → Nutrient availability is low at the bottom of the ocean as less sunlight penetrates into it.

Primary productivity is the rate at which plants make organic matter by photosynthesis. It depends on the light the plants receive and on the nutrients available to them.

Option (1) Nutrient availability is low at the bottom of the ocean as less sunlight penetrates into it. — Correct. Sunlight reaches only the upper layer of the ocean; deeper water is too dark for photosynthesis. The plants of the lit layer are short of nutrients such as nitrogen and phosphorus. The option names these two limits, light and nutrients, so it is the best choice.

Option (2) Marine diversity is highly concentrated around coastal areas of the ocean. — Incorrect. Life is richer near the coasts, but this only says where marine life is found. It does not explain why the open ocean makes so little.

Option (3) High salinity inhibits plant growth in the ocean. — Incorrect. Marine algae and phytoplankton are adapted to salt water. Salinity is not what keeps ocean productivity low.

Option (4) Low temperature at the bottom prevents plant and animal growth. — Incorrect. No plant grows at the dark ocean bottom in any case, because there is no light there. The limit is light, not temperature.

Note: NCERT gives the figures for the low productivity of the oceans but leaves the reason for discussion in class. The reason asked here is from beyond the NCERT textbook.

NCERT Extract:
Productivity – “… Net primary productivity is the available biomass for the consumption to heterotrophs (herbiviores and decomposers). Secondary productivity is defined as the rate of formation of new organic matter by consumers. Primary productivity depends on the plant species inhabiting a particular area. It also depends on a variety of environmental factors, availability of nutrients and photosynthetic capacity of plants. Therefore, it varies in different types of ecosystems. The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter. Of this, despite occupying about 70 per cent of the surface, the productivity of the oceans are only 55 billion tons. Rest of course, is on land. Discuss the main reason for the low productivity of ocean with your teacher.”