Which words in the sentence are the complete adjective clause? Ask him to tell the story, which is about wild ponies.

a. ask him

b. which is about wild ponies

c. to tell the story, which is about wild ponies

d. to tell the story

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.