Which of these is directly involved in transporting nutrients from the sugar solution into the yeast?

 Yeast to get glucose, the cells must use active transport to move glucose up its concentration gradient. Yeast cells take up glucose to use as their main source of energy. Typically, glucose concentrations outside yeast cells are lower than glucose concentrations inside the cells. 

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.