Which substance will not be returned to the ecosystem and recycled?

The diagram shows that the leaf loses oxygen, water vapour, and heat to the environment.

Oxygen is a gas released by plants during photosynthesis. It is then used by most living organisms for respiration and is continuously exchanged between the atmosphere, living organisms, and water bodies as part of the oxygen cycle. Therefore, oxygen is recycled.

Water vapour is released by plants through transpiration. This water vapour rises into the atmosphere, condenses to form clouds, and eventually returns to the Earth as precipitation (rain, snow, etc.), forming the water cycle. Therefore, water vapour is recycled.

Heat is a form of energy. Energy flows through an ecosystem, typically originating from the sun, being converted into chemical energy by producers, and then transferred through trophic levels. However, at each transfer, a significant portion of energy is lost as heat to the environment. This heat dissipates into space and cannot be re-captured and reused by the ecosystem in the same way that matter (like oxygen or water) is recycled. Energy flows in one direction and is ultimately lost as heat, not recycled.

Based on the analysis, oxygen and water vapour are recycled, but heat is not.

The final answer is $\text{A}$