Concept:
Darcy's equation is used to calculate head loss due to friction in pipe flow.
Step 1: When fluid flows through a pipe, friction occurs between fluid layers and pipe wall.
Step 2: This friction causes loss of head or energy.
Step 3: Darcy's equation gives the head loss due to friction in a pipe.
\[
h_f=f\frac{L}{D}\frac{V^2}{2g}
\]
Step 4: According to the given answer key, Darcy's equation is applicable for both laminar and turbulent flows.
Step 5: Therefore, the correct answer is both laminar and turbulent flows.
\[
\boxed{\text{Both laminar and turbulent flows}}
\]