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Correct answer: A. Mass
### Step 1: Understanding Inertia
Inertia is the inherent property of a body by virtue of which it resists any change in its state of rest or uniform motion in a straight line. It is not a force, but a property of matter.
### Step 2: Relate to Newton's First Law
Newton's First Law of Motion, often called the **Law of Inertia**, states that an object will remain at rest or move with constant velocity unless acted upon by an external force. The magnitude of this resistance depends on the amount of matter in the object.
### Step 3: Identify the Quantitative Measure
In classical mechanics, the mass ($m$) of an object is the quantitative measure of its inertia.
- A larger mass requires a larger force to achieve the same acceleration ($F = ma$).
- Inertia does not depend on the object's speed ($v$), its position, or the acceleration ($a$) it is currently experiencing.
Therefore, the inertia of a body depends solely on its **mass**.