**Core Concept**
The question is testing the knowledge of proper protocols for collecting and preserving DNA samples for forensic examination. DNA sampling involves careful handling to prevent contamination and degradation, ensuring the integrity of the DNA for accurate analysis.

**Why the Correct Answer is Right**
The correct answer is a buccal swab, also known as a buccal smear. This method involves gently rubbing the inside of the cheek with a cotton swab to collect epithelial cells, which contain DNA. The buccal swab is a non-invasive, relatively painless, and widely accepted method for collecting DNA samples, particularly in forensic and medical settings. This technique allows for the collection of a sufficient amount of DNA for analysis while minimizing the risk of contamination.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood sample - Blood is not the preferred choice for DNA sampling due to the presence of enzymes that can degrade DNA over time. Additionally, blood can be contaminated with external DNA sources.
**Option B:** Saliva sample - While saliva does contain DNA, it is not the preferred method for forensic DNA analysis due to the risk of contamination and the difficulty in collecting a sufficient amount of saliva for analysis.
**Option C:** Urine sample - Urine is not a suitable source for DNA sampling due to the presence of enzymes that can degrade DNA and the potential for contamination from external sources.

**Clinical Pearl / High-Yield Fact**
When collecting DNA samples, it is essential to handle the samples carefully to prevent contamination, as even a single foreign DNA molecule can compromise the integrity of the sample. A buccal swab is a reliable and widely accepted method for collecting DNA samples, making it a valuable tool in forensic and medical settings.

**Correct Answer: D. Buccal swab.**