Gross primary productivity is defined as:

Gross primary productivity (GPP) represents the total amount of energy that is captured and converted into organic matter by photosynthetic organisms, primarily plants, within an ecosystem. It is a measure of how much solar energy is transformed into chemical energy through the process of photosynthesis. This includes all of the energy produced before any is used by the organisms for their own metabolic needs, such as respiration.

While there are other ecological concepts that relate to energy flow, B accurately reflects the definition of GPP because it encompasses the entirety of the energy converted by plants during photosynthesis. Understanding GPP is crucial in studying energy transfer within an ecosystem, as it sets the baseline for how much energy is available to support all higher trophic levels, including herbivores and carnivores.

The other choices focus on different aspects of productivity and energy flow. Some refer to energy losses or availability after certain processes occur, like respiration or herbivory, and do not capture the initial conversion of energy by plants, which is the essence of gross primary productivity.