**Core Concept**
DNA fingerprinting is a technique used to identify individuals based on the unique patterns of their DNA. This method involves analyzing specific segments of DNA, known as variable number tandem repeats (VNTRs), which exhibit polymorphism among individuals. The process relies on the principle of genetic variation, allowing for the distinction between individuals.

**Why the Correct Answer is Right**
RBCs (Red Blood Cells) lack a nucleus, which contains the majority of the cell's DNA. As a result, RBCs cannot be used for DNA fingerprinting, as the necessary DNA material is absent. In contrast, cells with nuclei, such as WBCs (White Blood Cells), saliva, and spermatozoa, contain sufficient DNA for analysis.

**Why Each Wrong Option is Incorrect**
**Option B:** WBCs can be used for DNA fingerprinting due to their nuclear content, which provides the necessary DNA for analysis.
**Option C:** Saliva contains epithelial cells, which have nuclei and can be used for DNA fingerprinting.
**Option D:** Spermatozoa, despite lacking a nucleus, can be used for DNA fingerprinting through the analysis of their mitochondrial DNA.

**Clinical Pearl / High-Yield Fact**
When collecting DNA samples for forensic analysis, it's essential to consider the presence of epithelial cells, which can be found in various bodily fluids, including saliva. These cells can serve as an alternative source of DNA when other samples, such as blood or tissue, are not available.

**✓ Correct Answer: A. RBC. RBCs lack a nucleus, making them unsuitable for DNA fingerprinting.**