Final answer:
In the sentence 'Harry Potter, a fictitious character who was a wizard, had many adventures with Hermione and Ron,' the appositive phrase is 'a fictitious character who was a wizard,' which provides additional information about 'Harry Potter.'
Step-by-step explanation:
The appositive phrase in the sentence provided is a noun or noun phrase that renames or clarifies another noun next to it. In the sentence 'Harry Potter, a fictitious character who was a wizard, had many adventures with Hermione and Ron,' the appositive phrase is 'a fictitious character who was a wizard'. This phrase adds additional information about 'Harry Potter.' Options 'Harry Potter' and 'with Hermione and Ron' cannot be appositives in this sentence because they do not rename or clarify another noun. Meanwhile, the phrase 'who was a wizard' gives extra detail but does not alone represent the full renaming or clarification of 'Harry Potter.' Hence, the correct answer is d. 'a fictitious character who was a wizard'.