Why must the drilled hole be filled with drilling mud when tripping the drill string out of the hole?

Correct Answer: Option B — To prevent reduction of fluid head on the formations

Maintaining a full hole during tripping is vital to preserve the hydrostatic pressure exerted by the mud column. If the fluid level drops, the hydrostatic head decreases, potentially allowing formation fluids to enter the wellbore. Constant monitoring and filling ensure the well remains in an overbalanced, safe condition.