Molecules from which oxygen evolves during photosynthesis.

Explanation

**Step-by-Step Solution:**

1. **Understanding Photosynthesis**: Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy, using carbon dioxide and water to produce glucose and oxygen.

2. **Role of Water**: During photosynthesis, water (H₂O) is a crucial reactant. It undergoes a process called photolysis, where it is split into hydrogen (H) and oxygen (O) molecules.

3. **Photolysis of Water**: The breakdown of water into hydrogen and oxygen occurs in the presence of sunlight. This process is specifically known as photolysis.

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