Plants store energy as starch, a polysaccharide of glucose units. This serves as their primary energy reserve.
Step 1: Starch's Role.
Starch acts as the principal energy reserve in plants, stored in organs like roots, tubers, and seeds. It can be reconverted to glucose when required.
Step 2: Option Analysis.
- Option 1: Starch: Accurate; starch is the storage carbohydrate in plants.
- Option 2: Glycogen: Incorrect; glycogen is the glucose storage form in animals, not plants.
- Option 3: Cellulose: Incorrect; cellulose is a structural carbohydrate in plant cell walls, not for storage.
- Option 4: Glucose: Incorrect; glucose is an immediate energy source, not a storage form in plants.
Step 3: Determination.
Starch is the correct storage carbohydrate for plants.
Final Answer: \[\boxed{\text{Starch is the correct answer.}}\]