Step 1: Recall the meaning of inundation.
In underground mining, inundation is the uncontrolled entry of large quantities of water into mine workings.
Step 2: Identify the source of inundation.
The water generally enters from old abandoned workings, aquifers, or nearby surface water bodies.
Hence,
\[
\boxed{\text{Sudden influx of water from old workings or surface sources}.}
\]
Therefore,
\[
\boxed{(A)}
\]
is the correct answer.