How do these differences relate to the functions of these two similar epithelium?

Simple squamous is thin, to ease diffusion of gases (lungs) or other things, into the blood. Simple cuboidal is not very much more thick, but it usually has ceullular functions. Stratified squamous has many layers, and usually covers organs that go through a lot of friction and sloughs off, (skin). Stratified cuboidal has many layers for the same reason, but usually has more of a function than just protection, such as performing cellular functions. And when I say cellular functions I mean producing hormones, sweat and oil. Glands might be lined with them. Simple columnar and stratified columnar have a lot of surface area, which are great in the intestines, where we absorb food into the blood.

That is true tge cactus as a structural adaptation to its envirinment has it leaves reduced to spines. but this doesnt mean it cant produce food it produces food because of the presence of chlorophyll in its stem

C. a mature ovule of a plant.