You are correct - the blank should be filled with an adjective and not a noun, therefore la cortesía is out.
To figure out which gender the adjective must take, take a look at the subject of the sentence. The subject here is un huésped; it contains the indefinite article un. The definite article that goes with un is el, which we know is masculine (feminine definite is la, and feminine indefinite is una).
Therefore, the adjective must be masculine - respetuoso.
In a different scenario, where someone is saying this sentence to a female, they might say Es muy importante ser una huésped respetuosa.
