**Core Concept**
Celiac disease is an autoimmune disorder characterized by small intestinal mucosal damage in response to gluten ingestion. The diagnosis of celiac disease relies on a combination of clinical presentation, serological tests, and histological examination of intestinal biopsies. Circulating antibodies play a crucial role in the serological diagnosis of celiac disease.

**Why the Correct Answer is Right**
The serum tissue transglutaminase antibody (tTGA) has been widely recognized as the most sensitive and specific marker for celiac disease. This is because tTGA reacts with the tissue transglutaminase enzyme, which is responsible for deamidating gluten peptides, making them more immunogenic. The presence of tTGA antibodies indicates an autoimmune response against the gut epithelium, suggesting active celiac disease. In contrast, other markers like endomysial antibodies (EMA) and gliadin antibodies (GA) have lower sensitivity and specificity.

**Why Each Wrong Option is Incorrect**
**Option A:** Anti-endomysial antibodies (EMA) are highly specific for celiac disease but have lower sensitivity compared to tTGA. They are useful in confirming the diagnosis but may not detect all cases, especially those with mild or latent disease.
**Option B:** Gliadin antibodies (GA) are not specific for celiac disease and can be found in various gastrointestinal disorders, making them less useful for diagnosis.
**Option C:** Anti-gliadin antibodies (AGA) are also not specific for celiac disease and can be found in individuals with gluten sensitivity or wheat allergy, making them less reliable for diagnosis.

**Clinical Pearl / High-Yield Fact**
It's essential to note that a combination of serological tests, including tTGA and EMA, along with histological examination of intestinal biopsies, is the most reliable approach for diagnosing celiac disease.

**Correct Answer:** C. Tissue transglutaminase antibody (tTGA)