<p>Marketing specialists have conducted extensive research on <span style="text-decoration:underline;">what</span> attracts customers to a particular product.</p><p>The correct answer is (D) what.</p><div data-answer="d"><span style="font-size: 13.008px;">In this sentence, the phrase "attracts customers to a particular product" is the object of the preposition "on". The word that best fits in this context to introduce the clause that describes the object of the research is "what". "What" is used to introduce a clause that functions as a noun and represents an unknown or unidentified thing. In this case, the research is focused on identifying the factors that attract customers to a particular product. Therefore, the correct answer is (D) what.</span><br></div><p></p>