The form of carbohydrate that is synthesized in plants is glucose. Glucose is a simple sugar and is considered the primary source of energy for all living organisms. It is produced during the process of photosynthesis, where plants convert sunlight into energy. Glucose is then used by plants for various purposes, such as cell respiration, growth, and the synthesis of other complex carbohydrates like starch and cellulose.

Starch, which is mentioned in option 1, is actually a storage form of glucose in plants. It is composed of long chains of glucose molecules and is used by plants to store excess glucose for later use.

Fructose, mentioned in option 3, is another type of sugar but is primarily found in fruits and honey. It is not synthesized by plants.

Cellulose, mentioned in option 4, is a complex carbohydrate that forms the structural component of plant cell walls. It is composed of glucose molecules and provides rigidity and support to plant cells. However, it is not the form of carbohydrate primarily synthesized by plants.

Therefore, the correct option is 2 - Glucose.