mellifluous:sound::potable : water
Explanation: Mellifluous means musical, referring to the quality of sound. Hence, we must look for an adjective that would stand as a quality for the other word.
(a) saline : saltiness [Saline is not a quality if saltiness. They're synonyms.]
(b) dormant : bed [Dormant describes animals, not a bed.]
(c) potable : water [Potable means safe to drink. It may be a quality of water. Hence, it fits perfectly in the analogy.]
(d) voracious : food [Voracious describes a person or an animal. It does not refer to food.]