Step 1: Recall the purpose of disinfection.
Disinfection destroys disease-causing microorganisms present in treated water.
Step 2: Identify the common method.
The most widely used method of water disinfection is chlorination, i.e., passing chlorine into water.
Hence,
\[
\boxed{\text{Passing chlorine}}
\]
is the correct answer.
Therefore,
\[
\boxed{(A)}
\]
is the correct answer.