To solve this problem, we need to analyze each statement about viroids, viruses, and prions.$\newline$Let's evaluate each option:$\newline$• Option 1: Viroids lack a protein coat.$\newline$ - Viroids are indeed small, circular RNA molecules that do not have a protein coat.$\newline$ - This statement is correct.$\newline$• Option 2: Viruses are obligate parasites.$\newline$ - Viruses require a host cell to replicate, making them obligate parasites.$\newline$ - This statement is correct.$\newline$• Option 3: Infective constituent in viruses is the protein coat.$\newline$ - The infective part of a virus is its genetic material (DNA or RNA), not the protein coat.$\newline$ - The protein coat, or capsid, protects the genetic material but is not the infective agent itself.$\newline$ - This statement is incorrect.$\newline$• Option 4: Prions consist of abnormally folded proteins.$\newline$ - Prions are infectious agents composed of protein in a misfolded form.$\newline$ - This statement is correct.$\newline$Therefore, the incorrect statement is Option 3, which claims that the infective constituent in viruses is the protein coat.