Question

A common ancestor is evident when comparing __________________ of different species in the phylum Chordata, especially in the early stages.
A) embryos
B) fossils
C) motility
D) protein structure

Answer 1

Phylum chordata: characterized by the presence of a "notochord", pharyngeal slits, post-a*** tail, and a hollow nerve chord associated with the organisms back(dorsal).  

A common ancestor is evident when comparing __________________ of different species in the phylum Chordata, especially in the early stages.
A) embryos - animals in the phylum Chordata have 4 features that appear some point in their development. Usually, this point occurs embryogenesis.
B) fossils - This is the only other semi-likely answer because one would be able to see animal structure. However, the phrasing of this questions doesn't allow for this to be the answer.  The question specifically mentions "early stages" which aren't possible in fossils. 
C) motility - Incorrect because motility does not show the physicical attributes associated with the phylum. 
D) protein structure - incorrect because motility does not show the physicical attributes associated with the phylum. 

The answer here must be embryos. 

Hope this helped!

Answer 2

A) embryos

The embryos in the early stages of chordates suggest a common ancestor. They share gill slits and a tail in the early embryo stage