Undoubtedly the answer is coral reef and after that comes equatorial rain forest in case of primary gross production.

Coral reefs have the highest rate of gross primary production among the given options.

Gross primary production (GPP) refers to the total amount of energy that is captured by autotrophs (organisms that can produce their own food through photosynthesis) in an ecosystem through the process of photosynthesis. It represents the total amount of energy available for the entire food chain in an ecosystem.

Coral reefs are highly productive ecosystems due to the presence of symbiotic algae called zooxanthellae. These algae live within the tissues of coral polyps and provide them with energy through photosynthesis. This symbiotic relationship allows corals to thrive in nutrient-poor tropical waters.

The high rate of gross primary production in coral reefs can be attributed to several factors:

1. Abundant sunlight: Coral reefs exist in shallow, clear waters, which allow for maximum penetration of sunlight. Sunlight is essential for photosynthesis to occur, and the availability of abundant sunlight in coral reef ecosystems provides a favorable environment for high rates of photosynthesis.

2. Nutrient recycling: Coral reefs have efficient nutrient recycling systems. The corals and other organisms in the reef continuously recycle and reuse nutrients, ensuring that they are not lost from the system. This recycling process allows for the sustained growth and productivity of the ecosystem.

3. Biodiversity: Coral reefs are home to a diverse range of species, including corals, fish, and invertebrates. This high biodiversity contributes to the overall productivity of the ecosystem. Different species have different ecological roles and functions within the reef, leading to a more efficient and productive nutrient cycling system.

4. Continuous growth: Coral reefs are constantly growing and building upon themselves. Corals secrete calcium carbonate skeletons, which form the structure of the reef. This continuous growth process requires a significant amount of energy, contributing to the high rate of gross primary production.

In conclusion, coral reefs have the highest rate of gross primary production among the given options due to factors such as abundant sunlight, efficient nutrient recycling, high biodiversity, and continuous growth.