Tertiary productivity
Tertiary productivity is not the correct answer for this question. Tertiary productivity refers to the rate at which energy is transferred and stored by organisms at the highest trophic level in an ecosystem. It is the energy captured by carnivores and omnivores from consuming other organisms. This is not relevant to the question being asked.

Primary productivity
Primary productivity is also not the correct answer. Primary productivity refers to the rate at which solar energy is converted into organic compounds through photosynthesis by autotrophic organisms such as plants, algae, and some bacteria. It is the energy captured by producers in an ecosystem. While this concept is related to the question, it does not specifically refer to the rate at which solar energy is converted and stored per unit area over a time period.

Gross primary productivity
Gross primary productivity (GPP) is the correct answer for this question. GPP is the total rate at which solar energy is converted into organic matter through photosynthesis by producers in an ecosystem. It represents the total amount of energy captured by producers per unit area over a time period. GPP includes all the energy produced by photosynthesis, but it does not account for the energy used by the producers themselves through respiration.

Net productivity
Net productivity is not the correct answer for this question. Net productivity refers to the rate at which energy is stored as biomass by producers after accounting for the energy used by the producers themselves through respiration. It represents the energy available for consumption by consumers in an ecosystem. While net productivity is related to the question, it does not specifically refer to the rate at which solar energy is converted and stored per unit area over a time period.

In conclusion, the correct answer is gross primary productivity (GPP) as it represents the rate at which solar energy is converted and stored by producers per unit area over a time period.