This question tests three small but common grammar points at once: choosing "a" versus "an" by sound, joining two adjectives correctly, and picking the right preposition to finish a fixed phrase. Go blank by blank instead of judging the whole sentence at once.
Filling the blanks in order gives "a; an; and; at," which is option (B), and reads naturally as: "People are crowding around a pit into which an elephant has fallen ... made to look up at a vast, curiosity-stricken crowd."
Let's summarize:
The correct sequence of words is "a; an; and; at."