Final answer:
Warm salt water is denser compared to cold fresh water.
Step-by-step explanation:
The denser of the two options, warm salt water or cold fresh water, would be warm salt water (a).
Density is the ratio of mass to volume. When temperature increases, the volume of a substance generally expands and the substance becomes less dense. Therefore, warm salt water would have a lower density compared to cold fresh water. Additionally, the salt content does not affect the density significantly.