The best option to fill in the blank is C. emphatic.
The sentence would read: "The prince is emphatic in his plans for the party in the abbey." This suggests that he is expressing strong assurance or emphasis regarding his plans.
The other options do not fit as well in this context:
- A. sedate suggests calmness, which doesn't fit well with planning a party.
- B. propriety refers to proper behavior, not directly related to planning.
- D. dauntless means fearless, which is not specifically relevant to planning a party.