Which of the 18 genes required for the formation of the head capsid, is the structural gene for the protein subunit?

Answer: Option C

1. The question pertains to the genes involved in the formation of the head capsid of certain organisms, such as bacteriophages.

2. Among the 18 genes required for this process, Gene 26 is identified as the structural gene responsible for encoding the protein subunit that forms the head capsid.

3. The head capsid is a crucial part of the viral structure that protects the genetic material and plays an essential role in the viral lifecycle.

4. The other options are incorrect because they do not represent the gene responsible for the structural protein subunit of the head capsid.

Thus, the correct answer is Gene 26.