The Experiment:
In this experiment, mature leaves on a plant were enclosed in a transparent bag that contained radioactive CO2. The aim was to determine the distribution of radioactivity within the plant after a fixed period of time.

Explanation:
The correct answer is option 'A' - actively growing leaves. This is because actively growing leaves are the primary sites of photosynthesis and carbon fixation in the plant. Let's understand why this is the case:

1. Photosynthesis:
Photosynthesis is the process by which plants convert carbon dioxide (CO2) and water into glucose and oxygen, using energy from sunlight. This process primarily occurs in the chloroplasts of the leaf cells, specifically in the mesophyll cells.

2. Carbon Fixation:
During photosynthesis, plants take in CO2 from the atmosphere and fix it into organic molecules, such as glucose. This fixation of carbon occurs during the Calvin cycle, which takes place in the chloroplasts of the leaf cells. The fixed carbon is then utilized for various metabolic processes and growth.

3. Actively Growing Leaves:
Actively growing leaves have a high rate of photosynthesis and carbon fixation as they require energy and building blocks for growth. These leaves have a higher number of chloroplasts and a larger surface area, which maximizes their capacity for photosynthesis.

4. Radioactive CO2:
In the experiment, the transparent bag contained radioactive CO2. This means that the carbon atoms in the CO2 were radioactive and emitted radiation. When the plant took in the radioactive CO2 during photosynthesis, the radioactive carbon atoms became incorporated into organic molecules, such as glucose.

5. Distribution of Radioactivity:
Since actively growing leaves are the primary sites of photosynthesis and carbon fixation, they would accumulate the highest amount of radioactivity from the radioactive CO2. This is because they are actively taking in CO2 and fixing carbon during the experiment. The radioactivity would be present in the organic molecules, such as glucose, produced through photosynthesis in these leaves.

Conclusion:
In conclusion, the maximum radioactivity would be found in the actively growing leaves of the plant. This is because these leaves are the main sites of photosynthesis and carbon fixation, where the radioactive CO2 is incorporated into organic molecules. The experiment helps in understanding the distribution of radioactivity within the plant and highlights the importance of actively growing leaves in carbon assimilation.