**Core Concept**
Blood pressure level is a measurement used to assess the force of blood against the arterial walls, typically recorded in millimeters of mercury (mmHg).

**Why the Correct Answer is Right**
Blood pressure is measured using a scale that compares the force of blood to a standard unit of measurement, allowing for accurate comparison and categorization of blood pressure levels. The scale used for blood pressure measurement is a ratio scale, as it has a true zero point (the absence of blood pressure) and equal intervals between consecutive units. This allows for precise calculations and comparisons, making it an essential tool in clinical practice.

**Why Each Wrong Option is Incorrect**
**Option A:** Nominal scale - This type of scale is used to categorize data without any inherent order or ranking, such as eye color or blood type. It does not apply to blood pressure measurement.
**Option B:** Ordinal scale - This scale ranks data in order, but the intervals between consecutive units are not necessarily equal. An example is the ranking of exam scores, where the difference between first and second place may not be the same as the difference between second and third place.
**Option C:** Interval scale - This scale has equal intervals between consecutive units, but it does not have a true zero point. An example is temperature in Celsius or Fahrenheit.

**Clinical Pearl / High-Yield Fact**
Remember that blood pressure measurements are typically recorded in millimeters of mercury (mmHg) and are used to categorize blood pressure levels into normal, prehypertension, stage 1 hypertension, and stage 2 hypertension.

**Correct Answer: C. Interval scale