Step 1: Recall enzyme immobilisation methods.
Common methods of enzyme immobilisation include:
\[
\begin{aligned}
&\text{Adsorption},
&\text{Covalent binding},
&\text{Entrapment},
&\text{Encapsulation},
&\text{Cross-linking}.
\end{aligned}
\]
Step 2: Identify the most widely used method.
Among these methods,
\[
\boxed{\text{Adsorption}}
\]
is the simplest, economical and most commonly employed technique for immobilising enzymes on solid supports.
Therefore,
\[
\boxed{(A)}
\]
is the correct answer.