Which one of the following about enzymes is true?: (A) High Km-High affinity, (B) Low Km-High affinity, (C) Non-competitive inhibition- Decreases Km, (D) Non-competitive inhibition – Increased Km
Answer is B. (Refer: DM Vasudevan, Textbook of Biochemistry, 7th edition, pg no: 59 - 60)
The Michaelis constant (km)

Km provides a measure of the affinity of the enzyme for its substrate.
It is equal to substrate concentration at which reaction velocity is equal to ½ Vmax.
Km = substrate concentration when reaction velocity is % Vmax.
Km is constant for any given enzyme.
Km is independent of substrate or enzyme concentration.
The higher is KM, the higher is the concentration of the substrate required to reach ½ Vmax.
That is: at a lower affinity, a higher concentration of substrate is required to reach ½  Vmax.
↓ Km = ↑ affinity.