The cactus fondly known as “Mother-in-Law’s Chair” is

Correct option is D

“Mother-in-Law’s Chair” is a common ornamental name used in horticulture.
It refers to a round, spiny, barrel-shaped cactus.
Echinocactus grusonii is popularly known by this name. It has prominent ribs and sharp spines.
Lithops are stone plants, not barrel cacti. Hence, the correct answer is Echinocactus.
Therefore, option (d) is correct.