Dear friends, see the question and the options clearly.As they have said in the question, the experiment is on how overcrowding affects plant growth.For that, the conditions in which the plant(s) are growing should be the same. That means that the amount of sunlight, amount of water, amount of soil and condition of soil should be the same, whereas the number of plants growing in one pot, should not..As we are assured in the question, they are already giving the same amounts of sunlight and water to the plants.We only have to see that the amount and condition of soil is the same, and that, my friends, is only in option 'C'.So, that i.e option 'C', is the correct answer.