To convert a primary amine into a quaternary ammonium salt, three moles of alkyl halide are typically required. The reaction follows these steps:
1. First alkylation: A primary amine reacts with one mole of alkyl halide to form a secondary amine.
2. Second alkylation: The secondary amine reacts with another mole of alkyl halide to form a tertiary amine.
3. Third alkylation: The tertiary amine then reacts with a third mole of alkyl halide to form a quaternary ammonium salt.
Therefore, 3 moles of alkyl halides are needed to completely convert the primary amine to a quaternary ammonium salt.
Thus, the correct answer is (C) 3.