The absence of which of the following structures leads to the formation of naked seeds?

Naked seeds are characteristic of gymnosperms, where seeds develop without an ovary wall (fruit). The integument becomes the seed coat, and the nucellus provides nutrition to the developing embryo - both structures are present in gymnosperm seeds. The absence of the ovary wall is what makes the seed 'naked' and exposed, unlike angiosperms where the ovary develops into a fruit enclosing the seeds.