Adjective clauses provide information about some noun in the sentence, which is the same function as adjectives.
So an adjective would be "to tell the interesting story", where interesting gives some information about the story.
Similarly, "which is about wild ponies" is the whole adjective phrase - and this is the correct answer- gives some information about the story.
He was bellowing into the phone, giving orders to one of his employees.
She agreed it was time to beseech the former FBI agent for help.
The enemy soldiers were completely annihilated.
