The hydrophilic heads of the outer layer face the ?

The hydrophilic heads of the outer layer face the outside of the cell

the cladogram shown here shows similarities and differences among several classes of vertebrates. anatomically, the four have limb structure as their common ancestor. we might separate the mammals from the other three classes because they a) divergent; do not lay eggs b) identical; are warm-blooded c) varied; a four chambered heart d) homologous; have mammary glands

what are the awnser choices? i mean, without choices, it is an incompleat