Pressure in trapped air equals atmospheric plus hydrostatic pressure from water height

Steps:

• Total pressure on trapped air = atmospheric pressure + ρgh, where h is water column height above air.
• Tubes P and R have identical water column heights, so same hydrostatic pressure.
• Tube Q has a different (likely shorter) water column height, resulting in lower pressure.
• Thus, trapped air pressures equal in P and R, differing from Q.

Why A is correct:

• Hydrostatic pressure formula P = ρgh shows equal h in P and R yields equal total pressure.

Why the others are wrong:

• B: Q's pressure is actually lowest due to shorter water column.
• C: R's pressure equals P's, not greatest.
• D: P's pressure equals R's, not greatest.

Final answer: A