**Core Concept**
Alcaftadine is an antihistamine used in the treatment of allergic conjunctivitis. Its mechanism of action involves selective antagonism of the histamine H1 receptor, which plays a crucial role in mediating allergic responses in the eye.

**Why the Correct Answer is Right**
Alcaftadine is a non-sedating antihistamine, and its efficacy is attributed to its ability to effectively bind to the H1 receptor, thereby blocking the action of histamine. This results in reduced vascular permeability, decreased mucus production, and alleviation of itching and redness associated with allergic conjunctivitis. The specific concentration of alcaftadine used in clinical trials is critical in determining its efficacy and safety profile.

**Why Each Wrong Option is Incorrect**
**Option A:** 0.1% - This concentration is too low to effectively bind to the H1 receptor and produce significant therapeutic effects.
**Option B:** 1.0% - This concentration is too high and may lead to increased side effects such as dryness and blurred vision.
**Option C:** 0.01% - This concentration is much lower than the effective concentration and would likely result in inadequate therapeutic effects.

**Clinical Pearl / High-Yield Fact**
When prescribing alcaftadine, it's essential to note that its effectiveness is dose-dependent, and the recommended concentration is 0.25%. This concentration provides an optimal balance between efficacy and safety.

**Correct Answer: C. 0.01%**