The by - product of photosynthesis is

Photosynthesis is a process through which plants make their food on their own by converting solar energy into chemical energy and then storing this energy in the bonds of carbohydrates.

These carbohydrates are made from the raw materials carbon dioxide and water.

The by-product formed with carbohydrates is oxygen in photosynthesis.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O_2.

Hence option D: O₂ is the final answer.