When a composer sets a single syllable of a word to several notes of music, he or she is using style.
a. melismatic
b. syllabic
c. plainchant
d. cadence

Since "melisma" is a term used for singing of a single syllable over a succession of several musical notes, the answer is A, melismatic. B cannot be correct, because in syllabic singing, one syllable refers to a single note, not several. "Plainchant" is an unaccompanied melodic line. And "cadence" means at least two chords that concludes a piece of music.