**Core Concept**
The most common phobia is a type of anxiety disorder characterized by an excessive or irrational fear of a specific object, situation, or activity. This phobia can cause significant distress and impairment in daily life.

**Why the Correct Answer is Right**
The most common phobia is a well-documented phenomenon in the field of psychology. According to various studies, the most prevalent phobia is a fear of spiders (arachnophobia). This phobia is thought to be an evolutionary remnant, as our ancestors had a natural fear of venomous spiders. The fear response is mediated by the amygdala, a structure in the brain that processes emotions, particularly fear. The amygdala sends signals to other brain regions, leading to the release of stress hormones like adrenaline and cortisol.

**Why Each Wrong Option is Incorrect**
* **Option A:** Social phobia is a common anxiety disorder, but it is not the most prevalent phobia. Social phobia involves a fear of social situations, fear of being judged or evaluated by others, and fear of embarrassment.
* **Option B:** Agoraphobia is an anxiety disorder characterized by a fear of being in crowds, public places, or situations where escape might be difficult. While it is a common anxiety disorder, it is not the most common phobia.
* **Option C:** Claustrophobia is a specific phobia involving a fear of enclosed or small spaces. While it is a well-known phobia, it is not the most common phobia.

**Clinical Pearl / High-Yield Fact**
The most common phobia can be treated with cognitive-behavioral therapy (CBT), a type of psychotherapy that helps individuals change their thought patterns and behaviors. CBT has been shown to be effective in reducing symptoms of phobias, including arachnophobia.

**Correct Answer: C. Arachnophobia. Fear of spiders.**