## Core Concept
The question pertains to the types of RNA and their characteristics, specifically focusing on the presence of abnormal purine bases. Transfer RNA (tRNA) is known for containing modified or abnormal bases, which are crucial for its structure and function.

## Why the Correct Answer is Right
tRNA, or **transfer RNA**, contains several modified bases, including abnormal purine bases like **N6-isopentenyladenine** and **queuosine**. These modifications are essential for the proper folding of tRNA molecules, their stability, and their interaction with ribosomes and other factors during protein synthesis. The presence of these modified bases is a hallmark of tRNA and distinguishes it from other types of RNA.

## Why Each Wrong Option is Incorrect
* **Option A:** Messenger RNA (mRNA) primarily carries genetic information from DNA to the ribosome for protein synthesis. While mRNA does contain modified bases, such as **N6-methyladenosine**, it is not particularly known for abnormal purine bases to the extent that tRNA is.
* **Option B:** Ribosomal RNA (rRNA) makes up a large part of the ribosome and is involved in the catalysis of peptide bond formation. rRNA molecules do contain some modified bases, but they are not as extensively modified with abnormal purine bases as tRNA.
* **Option D:** Small nuclear RNA (snRNA) is involved in RNA splicing and contains modified bases, but like mRNA and rRNA, it is not specifically noted for abnormal purine bases.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **tRNA modifications are critical for its function**, and defects in these modifications have been associated with various diseases, including certain types of cancer and neurological disorders. This highlights the importance of understanding RNA structure and function in clinical contexts.

## Correct Answer: C. tRNA.