Question

what is the most likely reason that horses and mountain goats have hooves?
they have homologous structures because they have a common ancestor.
they have analogous structures because they have a common ancestor.
these are vestigial structures in both animals because they are not used.
these are similar structures due genetic mutations.

Explanation:

Homologous structures come from a common ancestor. Horses and mountain goats having hooves fits this. Analogous structures (different ancestry, same function) don't apply here. Vestigial structures (unused) are wrong as hooves are used. Similar due to genetic mutations is less likely than common ancestry for homologous traits.

Answer:

They have homologous structures because they have a common ancestor.