**Core Concept**
Genomic imprinting is an epigenetic phenomenon where the expression of a gene is influenced by its parental origin, leading to differential expression based on whether the gene is inherited from the mother or the father.

**Why the Correct Answer is Right**
Genomic imprinting occurs due to the methylation of specific DNA sequences, which affects gene expression. This process is essential for normal development and growth. Imprinted genes are often involved in growth regulation, and their dysregulation can lead to various disorders, such as Prader-Willi syndrome and Angelman syndrome. The epigenetic marks are erased during gametogenesis, ensuring that the gene is expressed based on its parental origin.

**Why Each Wrong Option is Incorrect**
**Option A:** X-inactivation is a process where one of the two X chromosomes in female mammals is randomly inactivated to avoid a doubling of gene expression. While it affects gene expression, it is not related to the parental origin of the genes.
**Option B:** Gene expression can be influenced by various environmental factors, but differential expression based on parental origin is a specific phenomenon known as genomic imprinting.
**Option C:** RNA interference (RNAi) is a mechanism that regulates gene expression by degrading specific messenger RNA molecules. It is not directly related to the parental origin of genes.

**Clinical Pearl / High-Yield Fact**
Genomic imprinting is crucial for fetal development, as it ensures the proper regulation of growth and development. Disorders resulting from imprinted gene dysregulation often manifest in infancy or early childhood, highlighting the importance of this epigenetic mechanism.

**Correct Answer: C. Genomic imprinting.**