D. They help plants get nutrients and water form soill This is called mutualism.
The best answer in this case is C, "the researchers applied creativity to solve a problem in running an experiment".
Distributing the computing load across the global community by sharing processing power is a creative response to tackle the challenge in simulating protein folding for the experiment. This does not change or reduce the scientific merits of the experiment, so we can discount the first two answers (a & b). Answer d talks about well established scientific techniques, although the question wasn't really centered around the specific techniques, so it's not as relevant an answer as C.
Because, it is the process by which they make or produce their food with the help of sunlight, water and carbon dioxide.
If photsynthesis is not there then there would be no production of food and ultimatley plants will die.