Which characteristic describes sclerenchyma cells?

Sclerenchyma cells are indeed characterized by their non-living nature at maturity and their rigidity. These cells typically have thick, lignified cell walls that provide structural support to the plant. The lignin imparts strength and makes sclerenchyma cells particularly resistant to compression and bending, which is essential for the plant's overall integrity and support.

In contrast, flexible and living cells at maturity are more characteristic of parenchyma or collenchyma tissues. Parenchyma cells often serve functions related to storage and metabolic processes and can remain alive at maturity. The softness mentioned in the context of young plants typically applies to parenchyma, which can contribute to softer plant tissues. Additionally, while cellulose is indeed a significant component of many plant cell walls, including those of sclerenchyma, it is the lignification in these cells that distinguishes them specifically, making their walls rigid rather than just cellulose-based like other types of cells.