Formal.
As far as I know, there is no such thing as either standard or practiced diction. Informal is the way you communicate with your friends, for instance, which is not how you would write a thesis paper, legal document, etc.
Answer and Explanation:
The author clearly uses a third-person point of view, which can be seen due to the lack of first person pronouns. Whether his point of view is limited or omniscient, however, is impossible to tell just from this part. This passage focuses solely on Mina, and even with her the narrator is not revealing any thoughts. As for creating interest in the story, the author introduces a conflict right from the start: the mysterious red box at Mina's feet. This event makes readers curious as to who left it there and why, and what the box contains.
I'm pretty sure its A. were starting to think the cat had gotten your tongue