**Core Concept**
Deuterium oxide (D2O), also known as heavy water, is a stable isotope of water that contains deuterium (a heavy isotope of hydrogen) instead of regular hydrogen. It is used in various medical applications, including measuring body volume.

**Why the Correct Answer is Right**
D2O is used to measure total body water (TBW) volume, which is an essential parameter in assessing fluid status and electrolyte balance in the body. The process involves administering a known amount of D2O orally or intravenously, allowing it to distribute throughout the body, and then measuring its concentration in a blood or urine sample. The volume of distribution of D2O is directly proportional to the TBW volume, making it a reliable method for estimating this value.

**Why Each Wrong Option is Incorrect**
**Option A:** While D2O can be used to measure TBW, it is not typically used to measure blood volume directly. Blood volume can be estimated using other methods, such as dilution techniques with radioactive isotopes.
**Option B:** D2O is not commonly used to measure muscle volume specifically. While it can provide information on TBW, which includes muscle water, it is not a direct measure of muscle volume.
**Option C:** D2O is not typically used to measure bone volume. Bone volume can be estimated using other imaging modalities, such as X-ray or CT scans.

**Clinical Pearl / High-Yield Fact**
D2O dilution method is a reliable and non-invasive technique for estimating TBW, which is essential in clinical settings for assessing fluid status and electrolyte balance in patients.

**Correct Answer: C. Extracellular fluid volume.**