Stick with your original instinct on this one because unless the exact values are given, it is best to stick with density and forget bulk modulus all together... in terms of the MCAT only of course.

Sound travels slower in denser medium of the same phase (solid, liquid or gas) almost always. Bulk modulus changes with temperature and pressure as does density in this case so it would actually depend on the condition of the experiment as to whether salt water or fresh water wins out.

For the purposes of the MCAT, I would use common knowledge or common sense about density properties only as your foundation for comparing unless given a passage about bulk modulus and changes in bulk modulus vs temp.

The bulk modulus of salt water and fresh water is not the same

How and when can you tell which has the greater effect on speed of sound? athe bulk modulus or density? I know that within the same phase density has the greater effect i.e. speed of sound is greater in He than O2. But for this example, isn't freshwater and saltwater both in liquid phase, so shouldn't we only consider density? why is bulk modulus more important here?

for this question, i'm almost certain you need more information to answer it with confidence. it is not really common sense, with our humble physics abilities, to predict that adding salt to water would change bulk modulus.

speed is proportional to the square root of bulk modulus and inverse square root of density. if you double both, speed of sound remains the same.

How and when can you tell which has the greater effect on speed of sound? athe bulk modulus or density? I know that within the same phase density has the greater effect i.e. speed of sound is greater in He than O2. But for this example, isn't freshwater and saltwater both in liquid phase, so shouldn't we only consider density? why is bulk modulus more important here?

The formulae for speed of sound in a liquid and in solid rod are essentially the same, except that the liquid equation uses bulk modulus and the solid rod equation uses Young's modulus.