**Core Concept**
Non-coding RNAs (ncRNAs) are a class of RNA molecules that do not encode proteins but play crucial roles in various cellular processes. Unlike messenger RNA (mRNA), which carries genetic information from DNA to the ribosome for protein synthesis, ncRNAs perform a range of functions, including gene regulation, epigenetic modification, and catalysis.

**Why the Correct Answer is Right**
The correct answer is **D. All** because all the options listed (siRNA, miRNA, and tRNA) are types of non-coding RNAs. Small interfering RNA (siRNA) and microRNA (miRNA) are involved in RNA interference (RNAi), a process that regulates gene expression by silencing specific mRNAs. Transfer RNA (tRNA) is responsible for translating the genetic code into specific amino acids during protein synthesis. These ncRNAs do not encode proteins but play essential roles in various cellular processes.

**Why Each Wrong Option is Incorrect**
**Option A:** siRNA is indeed a non-coding RNA, but stating it as an option separate from the others would be incorrect because it is not the only type of non-coding RNA.
**Option B:** miRNA is also a non-coding RNA involved in RNAi, but like siRNA, it is not the only type of non-coding RNA.
**Option C:** tRNA is a non-coding RNA involved in protein synthesis, but the question is asking for non-coding RNAs in general, and tRNA is often considered a distinct category.

**Clinical Pearl / High-Yield Fact**
Non-coding RNAs are often referred to as the "dark matter" of the genome because they were previously underappreciated but now recognized as crucial regulators of gene expression.

**Correct Answer:**
✓ Correct Answer: D. All