Classifying things by observing can best be done using a

Classifying things by observation can best be done by using a  : Taxon

Taxon divided organisms into different groups based on their unique similarities, which is the best way to classify things if you're doing it by observation.

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.

answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.