**Core Concept**
The question is testing the understanding of radiation safety and measurement principles in medical imaging. Radiation dose measurement is crucial to ensure patient safety and optimize imaging protocols.

**Why the Correct Answer is Right**
A **Thimble chamber** is a type of ionization chamber used to measure radiation dose. It consists of a small, cylindrical chamber filled with air or gas, which is exposed to a known radiation field. The charge collected on the chamber walls is directly proportional to the radiation dose, allowing for accurate measurement. Thimble chambers are widely used in radiation therapy and diagnostic imaging to calibrate radiation dosimeters and ensure accurate dose delivery.

**Why Each Wrong Option is Incorrect**
**Option A:** A **Geiger counter** is a type of radiation detector that measures radiation intensity, not dose. It's used to detect and measure radiation levels in the environment.
**Option B:** A **Film badge** is a personal dosimeter that measures radiation exposure over time, but it's not a device for measuring dose in real-time or in a specific radiation field.
**Option C:** A **Radiation thermometer** is a device that measures temperature, not radiation dose. It's used in various medical and industrial applications.

**Clinical Pearl / High-Yield Fact**
When working with radiation, always use proper shielding and follow established protocols to minimize exposure. Remember the ALARA principle: "As Low As Reasonably Achievable" radiation exposure should be the goal in all radiological procedures.

**Correct Answer:** C. A thimble chamber is a device used to measure radiation dose.