The answer is C. biologists.
The farmers are the people who follow the policies in the cultivation of farm crops. They follow the policies made by the policy makers to ensure land sustainability. However they are not the people who investigate the cause behind land degradation.
The lobbyist are hired professionals by private policy making firms. They pursue the policy makers to accept their draft of policy or make the policy makers to come up legitimate policies to support a cause.
The biologists are the scientists that are investigating the cause behind the land degradation. They design the solutions to the problem faced by the farmers and conservationists. The policy makers should support the biologists in their research and listen to them for designing their policies.