Final answer:
Plants give off oxygen in the presence of light through the process of photosynthesis, which also produces glucose and is crucial for Earth's ecological balance.
Step-by-step explanation:
The gas that plants give off in the light is oxygen. This process, known as photosynthesis, involves plants taking in carbon dioxide and water and using the energy from light to produce glucose (a sugar) and oxygen. When sunlight is present, plants engage in photosynthesis and release oxygen, which is a vital component of Earth's atmosphere and essential for the respiration of most living organisms. The role of plants in replenishing the atmosphere's oxygen is a fundamental aspect of Earth's ecological balance and contributes to the planet's capacity to support life.