$(a)$ Why do sclerenchyma cells have a narrow lumen?
$(b)$ Where are these tissues present and why?

(N/A) Sclerenchyma cells have a narrow lumen because of the excessive deposition of lignin on their cell walls,which makes the walls extremely thick and reduces the internal space.
$(b)$ Sclerenchyma tissues are present in stems,around vascular bundles,in the veins of leaves,and in the hard covering of seeds and nuts. They are present in these locations to provide mechanical strength,rigidity,and structural support to the plant parts.