Forced drop shaft method employed in

Forced drop shaft method (also known as caisson sinking) is mainly used in conditions where there is heavy inflow of water into the shaft.

In such strata, conventional shaft sinking becomes unsafe because continuous water ingress can collapse the sides and interrupt excavation.

The method uses a heavy, watertight caisson that is gradually forced downward by its own weight while excavation proceeds inside it, ensuring stability and control over water entry.

Although it can pass through bands of loose and tough strata, that is not the primary reason for its use.

The most critical and defining condition for employing this method is heavy inrush of water, making Option C the correct answer.