Plants release energy through a process called respiration. Respiration is the process by which plants break down glucose (produced during photosynthesis) to release energy, water, and carbon dioxide. This energy is used for various metabolic processes within the plant, such as growth, repair, and reproduction.

Option 1: Photosynthesis is the process by which plants convert sunlight, carbon dioxide, and water into glucose and oxygen. While photosynthesis is essential for plant survival, it is the opposite of energy release; it is an energy-absorbing process.

Option 3: Transpiration is the process of water loss from plants through their leaves. Although transpiration is necessary for plant cooling and nutrient uptake, it does not directly release energy.

Option 4: Germination is the process by which a seed begins to grow into a new plant. Energy is required for germination, but it is provided by the stored reserves within the seed, not released by the plant.

Therefore, the correct answer is option 2, respiration.