Which substance gives strength and protection to plant cells?

Cellulose is the correct choice because it is a complex carbohydrate that forms the structural component of the cell wall in plant cells. This tough and rigid substance provides strength and support, allowing plants to maintain their shape and resist external pressures. The presence of cellulose in the cell walls is crucial for the plant's ability to grow upright and withstand various environmental factors, including wind and water pressure.

In contrast, chlorophyll is the pigment responsible for photosynthesis, allowing plants to capture sunlight, but it does not provide structural strength. Starch serves as an energy storage molecule in plants, storing glucose for later use, yet it does not contribute to the physical support or protection of the cell. Sugar, primarily a simple carbohydrate, is involved in energy production and metabolism rather than providing any structural integrity to the cells. Thus, cellulose is specifically tailored for the purpose of giving strength and protection to plant cells.