An adjective is a word that describes a noun. Adjectives provide additional information about people, places or ideas. In this example, the adjective "light" is used to give more information about the suitcase that Michael had been dragging through the airport earlier in the day. Adjectives are one of the main parts of speech of the English language.
I would say D. because all of the others are basically answering the entirety of the speech. If you would discuss ways to increase the literacy rates in the U.S. you would need to go over it last after you explain why the U.S. is so illiterate.