Prions consist of abnormally folded proteins.

Viroids lack a protein coat.

Viruses are obligate parasites.

Infective constituent in viruses is the protein coat.

The correct answer is option 'D' - "Infective constituent in viruses is the protein coat."

Explanation:

Prions consist of abnormally folded proteins:
- Prions are infectious agents that are composed solely of proteins.
- These proteins have an abnormal conformation and are capable of inducing normal proteins to adopt the abnormal conformation as well.
- This conformational change leads to the formation of aggregates and causes damage to the nervous system.

Viroids lack a protein coat:
- Viroids are small infectious agents that are composed solely of RNA.
- Unlike viruses, viroids do not have a protein coat or capsid.
- They are smaller and simpler than viruses and can cause diseases in plants.

Viruses are obligate parasites:
- Viruses are infectious agents that are composed of genetic material (either DNA or RNA) surrounded by a protein coat called a capsid.
- They are considered obligate parasites because they cannot replicate or carry out metabolic processes outside of a host cell.
- Viruses rely on the machinery and resources of the host cell to replicate and produce more virus particles.

Infective constituent in viruses is the protein coat:
- This statement is incorrect.
- The infective constituent in viruses is the genetic material (DNA or RNA), not the protein coat.
- The protein coat, or capsid, serves to protect the genetic material and facilitate its entry into host cells.
- Once inside the host cell, the viral genetic material takes over the cellular machinery to replicate and produce more virus particles.

In conclusion, the incorrect statement is that the infective constituent in viruses is the protein coat. The infective constituent in viruses is the genetic material (DNA or RNA).