## Core Concept
The ABO blood group system involves the presence or absence of specific antigens on the surface of red blood cells and other tissues. These antigens determine an individual's blood type according to the ABO classification. The ABO antigens are primarily found on red blood cells but can also be present in other body fluids and tissues.

## Why the Correct Answer is Right
The correct answer, **D. Synovial fluid**, is right because while ABO antigens are widely distributed on various cells and in body fluids, synovial fluid, which is found in the cavities of synovial joints, does not typically express ABO blood group antigens on its components. This is a key point in understanding the distribution and clinical relevance of ABO antigens.

## Why Each Wrong Option is Incorrect
- **Option A:** Red blood cells express ABO antigens on their surface, which is the basis for ABO blood typing. Therefore, this option is incorrect because it does indeed contain ABO antigens.
- **Option B:** Saliva can contain ABO antigens, particularly in individuals who are secretors, meaning they secrete ABO blood group substances into their bodily fluids. This makes option B incorrect.
- **Option C:** Plasma can also contain ABO antigens, especially in individuals who are secretors. These antigens can be present as soluble substances. Thus, option C is incorrect.

## Clinical Pearl / High-Yield Fact
A memorable point for exams is that about 80% of people are secretors, meaning they express ABO antigens in their bodily fluids like saliva, tears, and plasma, while the remaining 20% are non-secretors and do not express these antigens in their bodily fluids. This distinction can be clinically relevant in forensic science and transfusion medicine.

## Correct Answer: D. Synovial fluid.