Introduction:
In ecology, a consumer is an organism that obtains energy and nutrients by feeding on other organisms or organic matter. Consumers are an essential part of food chains and food webs, as they consume producers (plants or algae) or other consumers. They can be classified into different types based on their feeding habits and position in the food chain.

Explanation:
To determine which of the following options is not a consumer, we need to understand the concept of consumers and their characteristics.

A) Cow:
Cows are herbivorous animals that consume grass, plants, and other vegetation. They are primary consumers, as they directly consume producers (plants) to obtain energy.

B) Lion:
Lions are carnivorous animals that consume other animals for their nutrition. They are secondary consumers, as they prey on herbivores (such as zebras or antelopes) that consume plants.

C) Tree:
Trees are not consumers. They are producers in the food chain, as they convert sunlight, water, and carbon dioxide into energy through the process of photosynthesis. They are the primary source of food and energy for other organisms but do not consume other organisms themselves.

D) Rabbit:
Rabbits are herbivorous animals that consume plants and grass. They are primary consumers, as they directly feed on producers (plants) to obtain energy.

Conclusion:
Based on the given options, the organism that is not a consumer is Option C) Tree. Trees are producers in the food chain, converting sunlight into energy through photosynthesis, and are not involved in consuming other organisms for their nutrition.