Differential thermal expansion Steps:

• The flask and water are heated to a higher temperature.
• Both the glass flask and water expand due to thermal expansion.
• Water has a greater coefficient of volume expansion than glass.
• The water's volume increase exceeds the flask's, forcing water up the tube to Y.

Why D is correct:

• D states the water expands more than the flask; this follows from the volume expansion formula ΔV = V₀ β ΔT, where β_water (2.1 × 10⁻⁴/°C) > β_glass (≈ 2.7 × 10⁻⁵/°C).

Why the others are wrong:

• A: Glass expands on heating, it does not contract.
• B: Neither contracts on heating; both expand.
• C: This describes expansion but ignores the flask's expansion relative to water's greater expansion.

Final answer: D