Which one of these is not a fruit?

Squash, tomatoes, green beans all fall within the botanical category “fruit” which forms out of a plant’s flower.

In the center of the flower is the female part known as the ovary that becomes the seeds when fertilized. These seeds develop into a fleshy fruit. Biologically the function of the fruit is to continue the plant lineage through plant reproduction. The fleshy fruit is digested by animals and distributed through their waste product or scattered by wind to continue the life cycle.

Rhubarb, being a stem, falls into the botanical “all other” category. Botanically the term “vegetable” has no meaning. All non-fruit produce is classified by the part of the plant they are: stem, root, leaf etc.