## Core Concept
The question tests knowledge on the effects of ionizing radiation on the human body, specifically the dose required to cause death. Ionizing radiation can damage living tissues, leading to acute radiation syndrome (ARS), which can be fatal at high doses.

## Why the Correct Answer is Right
A dose of **500 rads (or 5 Gy)** of ionizing radiation to the whole body can lead to severe damage to the bone marrow, lungs, gastrointestinal system, and central nervous system. This dose is within the range that can cause severe ARS, leading to death within weeks due to infections, bleeding, and organ failure. The dose of 500 rads is considered lethal because it exceeds the body's ability to repair damage to rapidly dividing cells, such as those in the bone marrow and gut lining.

## Why Each Wrong Option is Incorrect
- **Option A:** 100 rads is a dose that can cause mild to moderate ARS but is generally not lethal. It can lead to nausea, vomiting, and some bone marrow suppression but is typically not fatal.
- **Option B:** 200 rads can cause more pronounced ARS symptoms, including significant bone marrow suppression and gastrointestinal symptoms, but it is less likely to be directly fatal compared to higher doses.
- **Option D:** 1000 rads, while certainly dangerous and potentially lethal, the question seems to seek a dose that results in death and among given options, 500 rads is specifically known for its lethal potential.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the LD50/30 (the dose lethal to 50% of a population within 30 days) for humans exposed to whole-body radiation is approximately **400-500 rads (4-5 Gy)**. This dose can cause severe ARS and death in a significant portion of exposed individuals.

## Correct Answer: C. 500.