Answer:
en
The word en is a preposition. A preposition is a word (also a group of words) we use to point out the way which other words are connected. They are used to show a relationship between two words or phrases, between nouns, between nouns and verbs, and between different parts of a sentence. So here en is used to indicate location translating as at (or in). Finally;
Él estaba en el museo de Antropología y se cayó por la escalera