Infectious proteins are present in

The correct option is Prions.

To understand why this is the correct answer, let us break down what each of the options represents:
1. Prions: These are abnormal, pathogenic agents that are transmissible and able to induce abnormal folding of specific normal cellular proteins called prion proteins. Crucially, prions are composed entirely of protein material without any accompanying nucleic acid (DNA or RNA). Therefore, they are literally "infectious proteins."
2. Gemini viruses: These are a family of plant viruses that have a genome consisting of single-stranded circular DNA. They have a protein coat (capsid), but their genetic and infectious material is DNA, not just protein.
3. Satellite viruses: These are subviral agents composed of nucleic acids (either RNA or DNA) that require the co-infection of a helper virus to replicate. They are not made solely of protein.
4. Viroids: These are the smallest infectious pathogens known, consisting solely of a short strand of circular, single-stranded RNA without any protein coat.

Thus, among all the options provided, only Prions represent infectious agents that are purely proteinaceous in nature.