11) nutrients enter a cell the concentration gradient by the process of

Nutrients enter a cell DOWN the concentration gradient by the process of DIFFUSION.

This process of absorption of digested nutrients mainly uses diffusion (only some times active transport), which is a kind of method of material transportation where the net movement is fluid will flow from a region of higher concentration to the region of lower concentration, due to its original random movement.
Therefore, this net movement can be described as down the concentration gradient, and this process is diffusion.

The correct answer is c.