If green plant cells are incubated with `O^(18)`- labelled water, which of the following molecuels will become radioactive when the cells are exposed to light?

Options

A. `O_(2)`
B. `CO_(2)`
C. `H_(2)O`
D. Sugar

Explanation

To solve the question, we need to understand the process of photosynthesis and the role of water in this process, particularly when using O^(18)-labelled water.

### Step-by-Step Solution:

1. **Understanding Photosynthesis**: Photosynthesis is the process by which green plants convert light energy into chemical energy, using carbon dioxide and water. The general equation for photosynthesis is:
\[ 6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2 \]
Here, water (H2O) is split into oxygen (O2) and hydrogen during the light-dependent reactions.

2. **Role of O^(18)-labelled Water**: When we use O^(18)-labelled water, we are introducing a form of water where the oxygen atom has a heavier isotope (O^(18)). This means that when the water is split during photosynthesis, the oxygen released will also be O^(18), making it radioactive.

3. **Identifying the Molecules**: The question asks which molecules will become radioactive when the cells are exposed to light. The options given are:
- A) Oxygen
- B) Carbon dioxide
- C) Water
- D) Sugar

4. **Analyzing Each Option**:
- **Oxygen (A)**: When water is split during photosynthesis, the O^(18) from the labelled water will be released as O2. Thus, oxygen will become radioactive.
- **Carbon dioxide (B)**: Carbon dioxide does not contain any oxygen from the labelled water; it is taken from the atmosphere and does not become radioactive.
- **Water (C)**: The water used in the process is being split, and while it contains O^(18), it is not the product released into the atmosphere.
- **Sugar (D)**: The sugar produced (glucose) does not directly contain the O^(18) from the water; it is formed from carbon and hydrogen, and thus will not be radioactive.

5. **Conclusion**: The only molecule that becomes radioactive when the cells are exposed to light is oxygen (O2).

### Final Answer:
The correct option is **A) Oxygen**.