The professor's lecture, though lengthy and complex, (a) was (b) ultimately (c) insightful, (d) providing students (e) with a deeper understanding of the subject matter.
Correct: A
The subject of the sentence is "lecture," which is singular. Therefore, the verb should be singular as well. "Was" is the correct singular form of the verb "to be" in the past tense. Thus, (A) is correct.