Step 1: Definition of genomics.
Genomics is the biological discipline dedicated to the comprehensive study of the genome, the entirety of an organism's genetic material. It entails the analysis of genetic structure, function, and interrelationships to gain a holistic understanding of genetic information.
Step 2: Explanation of other options.
Option (B): Incorrect. The study of heredity and variation falls under genetics, distinct from genomics.
Option (C): Incorrect. While genomics examines the genome, the definition centers on the study of genes within an organism.
Option (D): Incorrect. This describes epigenetics, which investigates gene-environment interactions, not the scope of genomics.
Step 3: Conclusion.
Genomics is precisely defined as the study of genes within an organism. \[ \therefore \text{The correct answer is: The study of genes within an organism.} \]