Final answer:
The complete adjective clause in the sentence is 'which is about wild ponies', which provides additional information about the noun 'story'.
Step-by-step explanation:
The complete adjective clause in the sentence provided is which is about wild ponies. An adjective clause is a group of words that contains a subject and verb and provides more information about a noun. This clause modifies the noun 'story' and tells us what kind of story it is. Therefore, option b is the correct answer. The subject of the adjective clause is 'which' and the verb is 'is' while 'about wild ponies' acts as the subject complement.