Step 1: Recall the operating characteristics of an axial flow fan.
The pressure (water gauge) developed and the quantity of air circulated by an axial flow fan are primarily governed by the tip speed of the blades.
Step 2: Identify the governing parameter.
Since blade tip speed determines the energy imparted to the air,
\[
\boxed{\text{Speed of the blade tips}}
\]
is the correct answer.
Therefore,
\[
\boxed{(D)}
\]
is the correct answer.