The correct answer is option 1 - Light received from the sun. In an ecosystem, the source of energy is primarily the sun. This is because sunlight is essential for the process of photosynthesis, which occurs in plants. During photosynthesis, plants use sunlight, carbon dioxide, and water to produce glucose, a type of sugar. This glucose is then stored as energy in the plants.

Option 2 - Sugar stored in plants is not the source of energy, but rather a product of photosynthesis. Plants store glucose as a way to store energy for later use.

Option 3 - Heat liberated during fermentation of sugars is not the main source of energy in an ecosystem. Fermentation is a process that occurs in the absence of oxygen, and it is not the primary source of energy in most ecosystems.

Option 4 - Heat liberated during respiration does not describe the source of energy in an ecosystem. Respiration is the process by which organisms release stored energy from glucose in order to perform functions such as growth, reproduction, and movement. However, respiration relies on the previously stored energy from photosynthesis, which ultimately comes from the sun.

Therefore, option 1 is the correct answer as sunlight is the main source of energy in an ecosystem.