## Core Concept
Casoni's test, also known as the Casoni test or specific intradermal test, is a diagnostic tool historically used for **echinococcosis** (hydatid disease), which is caused by *Echinococcus granulosus*. The test involves an intradermal injection of hydatid fluid; a positive reaction is indicated by immediate swelling and a wheal at the injection site.

## Why the Correct Answer is Right
The sensitivity of Casoni's test varies but has been reported to be around **50-80%**. This range reflects its moderate sensitivity in detecting individuals with hydatid disease. The test works by inducing an allergic reaction in individuals previously sensitized to *Echinococcus* antigens present in the hydatid fluid. A positive result indicates exposure and some level of immune response to the parasite.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not accurately represent the known sensitivity range of Casoni's test.
- **Option B:** Similarly, this option does not align with the documented sensitivity of the test.
- **Option D:** This option suggests a much higher sensitivity than what is generally reported for Casoni's test.

## Clinical Pearl / High-Yield Fact
A key point to remember is that while Casoni's test can be used as a diagnostic aid, it has largely been replaced by more specific and sensitive serological tests (like ELISA) and imaging techniques. However, it's still worth noting for its historical significance and in scenarios where such tests might be considered.

## Correct Answer: C.