The lecture was extremely __________ and informative.

The correct answer is B: useful. This sentence describes a lecture using two adjectives connected by "and." The second adjective, "informative," provides a strong clue about the nature of the lecture. "Informative" means providing a lot of useful information. Therefore, the blank requires an adjective that is either synonymous with "informative" or complements it in a positive way. "Useful" fits this description perfectly, as something that is informative is typically also useful. The combination "extremely useful and informative" creates a coherent and positive description of the lecture. Options A ("boring"), C ("confusing"), and D ("dull") all carry negative connotations. "Boring" and "dull" are synonyms, suggesting a lack of interest, while "confusing" implies a lack of clarity. These negative adjectives contradict the positive meaning of "informative." Therefore, "useful" is the only option that logically and semantically completes the sentence, maintaining a consistent positive tone and meaning.