Answer:
A. four boxes in a row with arrows between them
Explanation:
A metabolic pathway refers to a chain of reactions in which the product of the first reaction becomes the substrate for the next reaction and so on.
Among the given options, out of four boxes, the first box represents the starting substrate for the pathway and the last box represents the final product. Rest two boxes represent the intermediates formed during the pathway.
For example, A--> B---> C--->D
