D I believe, in the picture that is attached explains
B and C)
They often obtain most of their nutrients through the cell membrane as diffusion is able to take place passively and so at no energy loss to the organism, this can be essential for a single celled organism as they are able to produce a lot less energy due to having fewer mitochondria in comparison to multicellular organisms.
They also require few resources as single celled organisms are very small and so require only a small amount of raw resources for respiration because they have a small energy demand to meet.
