Final answer:
The slowest speed of sound will be in a medium with high density and low stiffness, option C, since high density slows sound transmission and low stiffness indicates higher compressibility which also slows down sound.
Step-by-step explanation:
The speed of sound in a medium is determined by the medium's rigidity (or stiffness) and its density. The more rigid the medium, the faster the sound travels because rigidity means less compressibility. Conversely, the greater the density of the medium, the slower the sound travels because more mass slows down the transmission of sound energy. Therefore, option C, High density, low stiffness, would create the slowest speed of sound as the high density would slow down the sound and low stiffness (high compressibility) would not aid in its quick transmission.