Sharks (a type of cartilaginous fish) and trout (a type of bony fish) both obtain oxygen from the water through their gills. Interestingly, an accurate phylogenetic tree including only sharks, trout, and humans (a type of mammal) would place humans as the sister group to trout! This shows that gills are a:

a. synapomorphy that distinguishes trout and humans from sharks
b. plesiomorphy shared by sharks and trout, but secondarily lost in humans
c. synapomorphy used to distinguish fishes from mammals
d. plesiomorphy revealing why humans are distantly related to trout

Synapomorphy used to distinguish fishes from mammals.

Explanation:

Synapomorphy is also known as derived characteristics of the species that has been present in the ancestor as well. This characteristics is mostly present in the most recent common ancestors.

The synapomorphy charcaters is genereally used to compare or distinguish between the two different species. The gills are synapomorphy characters that are derived in fishes and used for the comparison between mammals and fish as mammals have lungs for respiration.

Thus, the correct answer is option (c).  

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.