**Core Concept**
RNA molecules are modified to incorporate various nucleotide analogs, which play crucial roles in regulating gene expression, protein synthesis, and cellular metabolism. These modifications can significantly affect the structure, stability, and function of RNA molecules.

**Why the Correct Answer is Right**
Modified nucleotides are incorporated into RNA molecules through a process called post-transcriptional modification. These modifications can occur in the nucleus, during transcription, or in the cytoplasm, after transcription. For example, pseudouridylation involves the isomerization of uridine to pseudouridine, which is commonly found in transfer RNA (tRNA) molecules and is essential for their proper folding and function. Similarly, 5-methylcytidine (m5C) is a modified nucleotide found in various types of RNA, including mRNA, tRNA, and rRNA, and plays a crucial role in regulating gene expression and cellular metabolism.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the question.
**Option B:** While this option is related to RNA, it is not a modified nucleotide.
**Option C:** This option is not directly related to the question.

**Clinical Pearl / High-Yield Fact**
Modified nucleotides play a critical role in regulating gene expression and cellular metabolism. Understanding the types and functions of modified nucleotides is essential for understanding various biological processes and diseases.

**Correct Answer: C. Cytidine**