I think it’s NO CHANGE. All of the other ones doesn’t make sense.
I think it was a horse.... I don't know
“About Mars” is the Adjective Phrase.
Adjective phrase describes “what kind of which ones”
So for example here
“After school, I looked for books about Mars.”
“About Mars” describes the kind of books so that makes it an Adjective phrase.
I hope this helps.
