Differential thermal expansion Steps:

• Hands warm the trapped air faster than the glass flask.
• Air expands more than the flask initially, raising internal pressure above atmospheric and pushing water down from P to Q.
• Glass then expands as it warms, increasing flask volume.
• Total volume now exceeds air expansion, dropping pressure below atmospheric and pulling water up. Why D is correct:
• D states the air expands more than the flask first (pressure up), then flask expands more than air (pressure down), per Charles's law (gas volume ∝ temperature) and linear expansion coefficients (air expands ~10x more than glass per °C). Why the others are wrong:
• A: Flask expands with heat, does not contract.
• B: Air expands continuously with warming, does not contract.
• C: Sequence reversed; air expands more initially, not flask first. Final answer: D