Fossil $X$ is older than fossil $Y$. The most evident answer is:

• A
  Fossil $Y$ has got some of the vestigial organs which are functional in $X$
• B
  Fossil $Y$ has got homologous and analogous organs of fossil $X$
• C
  Fossil $X$ is found in deeper sedimentation than $Y$
• D
  Fossil $Y$ was found in a better state than that of $X$