Two students studying physiology taste a known bitter substance, and both report sensing bitterness. they then sample another substance. student a reports sensing both a bitter taste and a salty taste, but student b reports only a salty taste. what is the most logical explanation?

It seems that you have missed the necessary options for us to answer this question so I had to look for it. Anyway, here is the answer. Based on the given scenario above, the statement that would be the most logical explanation is this: Student A has a protein receptor capable of detecting a bitter molecule found in that substance, whereas student B lacks that particular protein receptor. Hope this helps. 

It's most likely choice a, it makes the most sense in my opinion.