Which of the following tastes may be detected at very low concentration?
sweet
umami
bitter
salty

The taste which we can detect already at very small concentrations is salty taste.

The reason why we have a special affinity for tasting bitter things. The reason why we can taste bitter things at extremely low concentrations is because potentially dangerous compounds can be bitter and it made evolutionary sense for organisms to adapt to taste bitter things.  

I beleve the answer is rotting carcass.

The remaining answer choices are all inorganic.

Hope this helps.