The correct answer is option 4: mature sclerenchyma cells have a lignified wall and are dead.

Sclerenchyma cells are a type of plant cell that provide support and strength to the plant. They have thick secondary cell walls that are composed of lignin, a complex polymer that provides rigidity and protection. As these cells mature, their primary cell wall is fortified with lignin, making it more rigid and durable.

Option 1 is incorrect because while cellulose is a major component of the cell wall in many plant cells, mature sclerenchyma cells have a secondary cell wall composed of lignin, not cellulose.

Option 2 is incorrect because it states that the lignified wall is living. However, sclerenchyma cells die as they mature and their cell walls become more rigid.

Option 3 is incorrect because suberized walls are characteristic of cork cells, not sclerenchyma cells.

It is important to note that while mature sclerenchyma cells are dead, their cell walls provide structural support for the plant.