**Core Concept**
The question is asking about the type of fluid that is most suitable for postmortem investigation, specifically in the context of forensic pathology. The underlying principle being tested is the ability to identify and collect evidence from a deceased individual, which requires a fluid that is stable, easily analyzed, and not easily contaminated.

**Why the Correct Answer is Right**
The correct answer is EDTA (Ethylene Diamine Tetraacetic Acid) plasma. EDTA plasma is the best fluid for postmortem investigation because it is a stable and easily analyzed fluid that can provide valuable information about the deceased individual's blood chemistry, including glucose, electrolyte, and drug levels. EDTA prevents clotting by chelating calcium ions, allowing for accurate analysis of blood components.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum - Serum is not the best choice for postmortem investigation because it can be easily contaminated and may not provide a accurate picture of the deceased individual's blood chemistry.

**Option B:** Whole blood - Whole blood is not ideal for postmortem investigation because it can clot and become difficult to analyze.

**Option C:** Urine - Urine is not the best choice for postmortem investigation because it can be easily contaminated and may not provide a accurate picture of the deceased individual's metabolic state.

**Clinical Pearl / High-Yield Fact**
When collecting fluid for postmortem investigation, it is essential to use a sterile needle and syringe to minimize the risk of contamination and to ensure that the fluid is collected from the correct location.

**Correct Answer: D. EDTA plasma. EDTA plasma is the best fluid for postmortem investigation because it is a stable and easily analyzed fluid that can provide valuable information about the deceased individual's blood chemistry.**