The correct answer is option 'D' - Roentgen.

Introduction:
Radiation is the emission of energy as electromagnetic waves or as moving subatomic particles, especially high-energy particles that cause ionization. It is crucial to measure the amount of radiation accurately for various purposes such as medical imaging, radiation therapy, nuclear power plants, and radiation safety.

Explanation:
The amount of radiation is measured by a unit called the Roentgen (R), named after the German physicist Wilhelm Conrad Roentgen, who discovered X-rays in 1895. The Roentgen is used to measure the ionizing radiation exposure in the air.

1. Importance of Measuring Radiation:
Radiation measurement is essential for several reasons:
- Radiation Safety: It helps in assessing and monitoring radiation levels to ensure the safety of workers and the general public.
- Medical Applications: Radiation measurements are used in medical imaging techniques like X-rays, CT scans, and mammograms to diagnose and treat diseases.
- Nuclear Power Plants: Radiation measurements play a vital role in monitoring radiation levels in nuclear power plants to ensure safe operations and protect workers.
- Research and Development: Radiation measurement is crucial for scientific research, studying the effects of radiation, and developing new technologies.

2. The Roentgen Unit:
The Roentgen is defined as the amount of radiation that produces one electrostatic unit of charge (either positive or negative) in one cubic centimeter of dry air at standard temperature and pressure. It quantifies the amount of ionizing radiation in the air.

3. Other Units to Measure Radiation:
While the Roentgen is primarily used to measure radiation exposure in the air, there are other units to measure different aspects of radiation:
- Rad (Radiation Absorbed Dose): It measures the amount of radiation energy absorbed by matter, typically in joules per kilogram.
- Gray (Gy): It is the SI unit of absorbed dose, equivalent to one joule of energy absorbed per kilogram of matter.
- Sievert (Sv): It measures the biological effect of radiation, taking into account the type of radiation and its impact on human tissues.

Conclusion:
The amount of radiation is measured using the Roentgen unit, which quantifies ionizing radiation exposure in the air. While other units like the Rad, Gray, and Sievert are used to measure different aspects of radiation, the Roentgen specifically focuses on the ionizing radiation in the air. Accurate radiation measurement is crucial for various applications, including medical imaging, radiation therapy, and ensuring radiation safety in different industries.