The answer is option 3. This is because when you change the volume of water in the test tube, the amount of air decreases. (You can’t increase the volume of water since it will overflow.) The oil at the top prevents excess surrounding air to enter to test tube for the plant to photosynthesise to produce oxygen (the bubbles form). The graph showed the decrease in variable Y when they change variable X so when there is lesser air dissolved in the water, the number of bubbles produced per minute decreases. For knowledge, option 1 is wrong as changing the amount of oxygen doesn’t affect the temperature. It is only when you change the temperature of water, then it affects the amount of oxygen. So the option is opposite. If you change the light intensity, the rate of photosynthesise should increase, however, the graph shows a decline so option 2 is wrong too. The last option is also wrong as the number of the plants doesn’t affect the light intensity (in this case).

Hope it helped you :)