The word that should be in bold is “Random”.
That being the case, the correct answer is D) an adjective.
In the part of the speech that says “the survey seems random but the results are invalid because the participants were screened”, the word in bold is an adjective.
In Grammatics, an adjective is a word that modifies a noun. The adjective modifies or decribes a noun adding characteristics to it regarding size, weight, color, shape, material, or age. In this case, the adjective “random” is modifying the noun “survey.”