**Core Concept**
The question is testing the understanding of gene editing techniques, specifically the role of a specific gene in one of these methods. Gene editing involves making precise changes to the DNA sequence of an organism. This can be achieved through various techniques, including CRISPR-Cas9, TALENs, and ZFNs.

**Why the Correct Answer is Right**
The CRISPR-Cas9 system relies on the presence of the Cas9 enzyme, which cleaves the DNA at a specific location. A mutation that completely disrupts the function of the *Cas9* gene would render the CRISPR-Cas9 system ineffective, as it would be unable to cleave the DNA. This is because the Cas9 enzyme is essential for the CRISPR-Cas9 system to function. Without it, the system would not be able to edit the gene.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because CRISPR-Cas9 is not based on the function of the *RNA-guided endonuclease*. While RNA-guided endonucleases are involved in the CRISPR-Cas9 system, the Cas9 enzyme is the key component that cleaves the DNA.

**Option B:** This option is incorrect because *guide RNA* is a crucial component of the CRISPR-Cas9 system, but it is not the gene that is being asked about. The guide RNA guides the Cas9 enzyme to the specific location in the genome where the edit needs to be made.

**Option C:** This option is incorrect because *DNA repair machinery* is necessary for the CRISPR-Cas9 system to function, but it is not the gene that is being asked about. The DNA repair machinery is involved in the process of repairing the DNA after it has been cleaved by the Cas9 enzyme.

**Option D:** This option is incorrect because *template DNA* is not a gene that is involved in the CRISPR-Cas9 system. Template DNA is used as a reference for the guide RNA to guide the Cas9 enzyme to the specific location in the genome where the edit needs to be made.

**Clinical Pearl / High-Yield Fact**
One important point to remember is that the CRISPR-Cas9 system is a powerful tool for gene editing, but it can also have off-target effects if not used carefully. This is because the Cas9 enzyme can cleave the DNA at multiple locations, leading to unintended changes to the genome.

**Correct Answer: C. CRISPR-Cas9 system relies on the presence of the Cas9 enzyme, which cleaves the DNA at a specific location.**