Step 1: Recall the meaning of slip.
Ideally, the air leaves the impeller exactly along the blade direction.
In practice, the flow lags behind the rotating blades because of a finite number of blades.
Step 2: Define the slip factor.
The slip factor accounts for the deviation of the actual exit whirl velocity from the ideal value.
Thus,
\[
\boxed{
\text{Slip factor accounts for the deviation of actual flow from the ideal blade direction at the impeller exit.}
}
\]
Therefore,
\[
\boxed{(B)}
\]
is the correct answer.