**Core Concept**
The question is testing the understanding of enzyme activity units, a fundamental concept in biochemistry. Enzyme activity is typically measured in units that reflect the amount of product formed or substrate consumed per unit time. The most commonly used unit of enzyme activity is the International Unit (IU), which is defined as the amount of enzyme required to catalyze the conversion of one micromole of substrate per minute.

**Why the Correct Answer is Right**
The International Unit (IU) is a widely accepted standard for expressing enzyme activity. It is based on the idea that one IU of an enzyme is equivalent to the amount of enzyme that catalyzes the conversion of one micromole of substrate per minute. This unit is useful for comparing the activity of different enzymes and for expressing the activity of a particular enzyme in a specific reaction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Unit of enzyme activity is not measured in terms of the enzyme's molecular weight or size. While the molecular weight of an enzyme can affect its activity, it is not a direct measure of enzyme activity.

**Option B:** This option is incorrect because the Katal (kat) is a unit of enzyme activity, but it is not the most commonly used unit. The Katal is defined as the amount of enzyme that catalyzes the conversion of one mole of substrate per second, which is a more extreme measure of enzyme activity than the International Unit.

**Option C:** This option is incorrect because the Molar (M) is a unit of concentration, not enzyme activity. While the concentration of an enzyme can affect its activity, the Molar is not a direct measure of enzyme activity.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that enzyme activity is often expressed in terms of the amount of product formed or substrate consumed per unit time, rather than the amount of enzyme present. This is because enzyme activity can be affected by many factors, including temperature, pH, and substrate concentration.

**Correct Answer: D. International Unit (IU)**