**Core Concept**
The human genome is the complete set of genetic instructions encoded in DNA, comprising all the genes, regulatory elements, and other functional sequences. The human genome is estimated to contain approximately 20,000-25,000 protein-coding genes, but this number can vary slightly depending on the source and the criteria used to define a gene.

**Why the Correct Answer is Right**
The Human Genome Project, completed in 2003, provided a comprehensive map of the human genome. The project estimated that the human genome contains around 20,000 to 25,000 protein-coding genes. These genes are responsible for encoding the proteins that perform a wide range of functions, from structural and enzymatic to regulatory and signaling. The exact number of genes in the human genome may vary slightly depending on the assembly used and the criteria for defining a gene.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the number of genes in the human genome is not in the range of tens of thousands. While there are approximately 20,000-25,000 protein-coding genes, non-coding regions and pseudogenes are also present.
**Option B:** This option is incorrect because the number of genes in the human genome is not in the range of hundreds of thousands. The estimated number of genes in the human genome is much lower than this.
**Option C:** This option is incorrect because the number of genes in the human genome is not in the range of millions. The estimated number of genes in the human genome is much lower than this.

**Clinical Pearl / High-Yield Fact**
The human genome is incredibly complex, with approximately 3 billion base pairs of DNA encoded in 23 pairs of chromosomes. Understanding the structure and function of the human genome is essential for advances in medicine and our understanding of human disease.

**Correct Answer: C. 20,000-25,000.**