Final answer:
The correct sentence is 'b) Tom Robinson, the defendant, seemed very trustworthy.' The phrase 'the defendant' is an appositive and should be enclosed with commas.
Step-by-step explanation:
The correct sentence is b) Tom Robinson, the defendant, seemed very trustworthy. When we include additional information about a person, place, or thing in a sentence, such as a title or a description, we often surround this information with commas. This additional information is called an appositive. In the given sentences, 'the defendant' is an appositive providing more information about 'Tom Robinson' and should be set off with commas before and after the phrase.