When an enzyme is competitively inhibited, which of the following changes occur?: (A) The apparent Km is unchanged., (B) The apparent Km is decreased., (C) Vmax is decreased., (D) Vmax is unchanged.
Answer is D. Features of noncompetitive inhibition:

Inhibitor have no structural resemblance to substrate.
Most irreversible (Except a few reversible non-competitive inhibition).
Excess substrate do not abolish the inhibition.
Km remains the same.
Vmax decreases.

 
Features of competitive inhibition:

Inhibitor will be structural analogue of substrate.
Reversible 
Excess substrate abolish the inhibition.
Km increases.
Vmax remains the same.