Final answer:
The speaker was astonished, doubtful, and terrified by Rev. George Bull's proposition, suggesting it was important but frightening work. Hence, the correct answer is option (B).
Step-by-step explanation:
The speaker seems to feel about the proposition that Rev. George Bull gave him as indicated by option B, which states that he thought it was important but frightening work.
The words "astonishment, doubt, and terror" suggest that while the speaker understands the significance of the task at hand, he is also overwhelmed by the weight of responsibility and potential implications of undertaking such a challenging and potentially controversial endeavor.