In the following sentence, which words form the compound subject? palms and cypress dotted the prairie and bordered the marsh. a. palms, cypress b. dotted, bordered c. prairie, marsh d, palms, prairie

A compound subject is a subject containing more than one word, usually connected by a conjunction 'and.' Having that in mind, the correct answer is A. palms, cypress. 
As you can see, there are two words making up the subject of the sentence, which is why the correct answer is A only. 

A. "Palms and cypress" is the subject of the entire sentence. As there are two nouns with a conjunction, they are by definition, compound.