Identify the part of speech of the underlined word: Floods leave people vulnerable to water-borne diseases.

The question is asking you to identify what part of speech the underlined word belongs to. In this case, the underlined word is "vulnerable". To find the answer, you need to think about what "vulnerable" is doing in this sentence. Is it describing a noun, like "people", or is it modifying a verb or adjective? In this case, "vulnerable" is describing the state of the people who are affected by floods. It is not a noun, verb, or adverb, but rather an adjective that is modifying the noun "people". Therefore, the correct answer is "Adjective".