The correct answer is: Phonetic
Phonetics is part of the science of vocal sounds, and explains the correspondence of sounds with symbols. A Phonetic word is one that sounds like it is spelled, such as: bed, pent.
There is an international phonetic alphabet that standardises certain symbols used to describe the sounds of words and letters.
Onomatopoeia is merely a word that describes a sound, such as: woof, purr, bark (as in dog), 'cough'.