The correct answer is option 'C' - NPP (Net Primary Productivity).

**Explanation:**

Net Primary Productivity (NPP) is the measure of the amount of organic matter produced by plants through photosynthesis after accounting for the energy used by the plants in respiration. It represents the available organic matter for herbivores in an ecosystem. Let's break down the concepts in more detail to understand why NPP is the correct answer.

1. **Primary Productivity (GPP)**: GPP stands for Gross Primary Productivity. It is the total amount of energy captured by plants through photosynthesis in a given time period. GPP represents the total organic matter produced by plants, including the energy used by the plants themselves during respiration.

2. **Respiration**: Respiration is the process by which plants and animals release energy from organic molecules to carry out their metabolic activities. Plants use a portion of the energy they capture through photosynthesis for their own growth, maintenance, and reproduction. This energy is lost as heat during respiration and is not available to herbivores.

3. **Net Primary Productivity (NPP)**: NPP is obtained by subtracting the energy used by plants in respiration (R) from the GPP. In other words, NPP represents the energy or organic matter available for consumption by herbivores and other heterotrophs.

NPP = GPP - R

NPP is the energy that is stored in plant tissues and is therefore available to herbivores as food. Herbivores consume plants and extract energy from the organic matter present in plant tissues.

To summarize, NPP represents the available organic matter for herbivores because it accounts for the energy used by plants in respiration, thus providing an estimate of the energy or biomass that herbivores can obtain from consuming plants. GPP, on the other hand, represents the total amount of energy captured by plants through photosynthesis, including the energy used by plants themselves. Secondary productivity refers to the production of organic matter by heterotrophs, such as herbivores, and is not directly related to the available organic matter for herbivores.