Substrate concentration at which Vi is half the maximal velocity attainable at a paicular concentration of enzyme is called as

**Core Concept**
The question is testing the understanding of enzyme kinetics and the concept of Michaelis-Menten constant (Km). Km is a measure of the substrate concentration at which the enzyme reaches half of its maximal velocity (Vmax). This concept is crucial in understanding how enzymes catalyze reactions and how their activity can be influenced by substrate concentration.

**Why the Correct Answer is Right**
The Michaelis-Menten constant (Km) is a mathematical constant that represents the substrate concentration at which the enzyme reaches half of its maximal velocity (Vmax). This is derived from the Michaelis-Menten equation, which describes the relationship between substrate concentration and enzyme velocity. At Km, the enzyme is operating at half of its maximum capacity, and it is the point at which the enzyme substrate complex is formed at a rate equal to the rate of product formation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not related to the concept of Km and enzyme kinetics.
**Option B:** This option is incorrect because Km is not a measure of enzyme affinity or binding capacity, but rather a measure of substrate concentration at which the enzyme reaches half of its maximal velocity.
**Option C:** This option is incorrect because Km is not a measure of the maximum velocity of the enzyme, but rather a measure of the substrate concentration at which the enzyme reaches half of its maximum velocity.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that Km is a measure of the substrate concentration at which the enzyme reaches half of its maximal velocity, and it is an important concept in understanding enzyme kinetics and the regulation of biochemical reactions.

**Correct Answer:** C. Km