**Core Concept**
Southern blot is a laboratory technique used to detect a specific DNA sequence in a sample. This technique involves the separation of DNA fragments by size, transfer of the fragments to a membrane, and subsequent hybridization with a labeled probe that is complementary to the target DNA sequence.

**Why the Correct Answer is Right**
The Southern blot technique is an adaptation of the Northern blot, which is used to detect RNA. In Southern blot, DNA is first digested with restriction enzymes to generate fragments of varying sizes. These fragments are then separated by size using gel electrophoresis. The separated fragments are then transferred to a membrane, where they are hybridized with a labeled probe that is complementary to the target DNA sequence. The probe binds to the target sequence, allowing for visualization of the target DNA.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Western blot is a technique used to detect proteins, not DNA. While Western blot also involves gel electrophoresis and transfer to a membrane, it uses antibodies to detect proteins rather than a labeled probe to detect DNA.
**Option B:** This option is incorrect because PCR (Polymerase Chain Reaction) is a technique used to amplify specific DNA sequences, but it is not used to visualize DNA.
**Option C:** This option is incorrect because Northern blot is a technique used to detect RNA, not DNA.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Southern blot is a laboratory technique used to detect specific DNA sequences, and it involves the use of restriction enzymes, gel electrophoresis, and hybridization with a labeled probe.

**Correct Answer: D. DNA**