Here it means specifically requested or ordered for use in the book.

"Commissioned" means "made to order, for money." A buyer "commissions" a work of art when he places an order for something to be made specially for him, for his purposes, and to his specifications. For example, an artist might paint a picture just because he wanted to, and then try to sell it. Or, a publisher might say to an artist, "We need a good book cover design. We would like to commission you to make one. It needs to be designed to reproduce well on a 6x9" cover; the design needs to be light-colored near the top so the title can be printed there; and we want it to illustrate the scene where Tom's friends are whitewashing the fence." According to the paragraph, the illustrations are reproductions of trade catalogues and advertisements from the time, so they were NOT specially commissioned for the book. "Vernacular" is an odd word to use, because the usual context is vernacular language--everyday language as spoken by the people, as opposed to literary language. Here, though, it just mean "everyday;" that is to say, "vernacular sources" means "everyday sources," and thus the correct answer is b.

endorsed