(A) describes something that’s essential for proper functioning (electrical power) and notes that generators are a backup way to provide this essential thing. But this isn’t analogous, because secondary substances aren’t a backup way to provide what primary substances provide.

This is the most analogous. Engines and transmissions are necessary for proper functioning of a car, just as primary substances are to plants. But other features, such as paint and taillights, give the car its superficial features (the look) as well as help protect the car (preventing rust), just as secondary substances give the plant superficial features (taste and smell) and help protect the plant (defense mechanisms).

(C) describes something that’s essential for proper functioning (mechanical components). But it then discusses electrical components, which are necessary to run the mechanical components. This isn’t analogous, because secondary substances aren’t necessary in order for the primary substances to work.

(D) discusses something necessary for the proper functioning of a train — a braking system. It then discusses two components of a braking system. But this isn’t analogous, because we want discussion of something that’s not essential to the proper functioning of a train, just as secondary substances are not essential to the proper functioning of a plant.

(E) discusses something necessary for the proper functioning of a word processor. But we don’t get a discussion of something that’s not necessary for computers to function as a word processor.