The correct answer is A carbon dioxide.
Photosynthesis requires carbon dioxide (CO₂) in order to produce glucose and oxygen (O₂) using sunlight. The bubbles observed on the plant's leaves are likely oxygen produced as a byproduct of the photosynthesis process. If there is not enough carbon dioxide in the water, the plant may not be able to perform photosynthesis efficiently, leading to fewer or no bubbles. Adding carbon dioxide to the water would increase the photosynthesis process and likely cause the bubbles to reappear.