Manometer measures pressure difference via mercury height.

Steps:

• Atmospheric pressure supports 760 mm Hg; manometer shows height difference due to P vs. atmosphere.
• Diagram indicates mercury 74 mm higher on open (right) side, so P < atmospheric pressure.
• Pressure difference h = 74 mm = atmospheric - P.
• P supports 760 mm - 74 mm = 686 mm Hg.

Why B is correct:

• B matches P = Patm - h, where h is the manometer reading, per the hydrostatic pressure formula ρgh.

Why the others are wrong:

• A: Subtracts 84 mm instead of 74 mm from 760 mm.
• C: Adds 76 mm to 760 mm, assuming P > Patm.
• D: Subtracts 124 mm from 760 mm, misreading the height difference.

Final answer: B