The professor a distinguished scholar in the field of astrophysics delivered a captivating lecture.
Correct: B
The phrase 'a distinguished scholar in the field of astrophysics' is an appositive that renames 'The professor.' It is non-essential information and should be set off by commas. Since the appositive is at the beginning of the sentence and modifies the subject 'The professor', it requires a comma after the subject and after the appositive phrase (which is implicitly at the end of the modifying phrase, right before the verb). Option B adds the necessary comma after 'professor' to begin the appositive phrase.