What type of cell wall is typically thicker and more rigid, found in mature plant cells?

The secondary cell wall is indeed the correct answer as it is typically thicker and more rigid than the primary cell wall. This structure is formed after the cell has reached maturity and is often composed of cellulose, lignin, and other polysaccharides, which contribute to its strength and durability. The secondary cell wall provides significant structural support, helping to maintain the integrity of the plant as it grows and matures.

In contrast, the primary cell wall is thinner and more flexible, allowing for growth and expansion of the plant cell during its early development. The middle lamella, on the other hand, is a pectin-rich layer that acts as a glue between adjacent plant cells, helping to adhere them together but does not contribute directly to the rigidity of individual cells. The term "protective layer" is vague and does not specifically refer to any recognized structural component of plant cells in this context. Therefore, the secondary cell wall stands out as the definitive structure that fits the characteristics of being thicker and more rigid in mature plant cells.