Final answer:
Commas should be added to the student's sentence to correctly read 'His apology, a very sincere one, was accepted warmly.' The review sentences provided also have corrections where commas are either added or removed for clarity and grammatical correctness.
Step-by-step explanation:
The sentence 'His apology, a very sincere one, was accepted warmly.' requires the addition of commas. Commas should be placed before and after the phrase 'a very sincere one' to set it apart from the rest of the sentence. The revised sentence would be: 'His apology, a very sincere one, was accepted warmly.'