Physical quantities are measurable properties with defined units

Steps:

• Define a physical quantity as a property like length or force that can be measured and expressed with a numerical value and unit.
• Evaluate option A: Metre is a unit of length, not the quantity itself.
• Evaluate option B: Percentage uncertainty describes error in measurement, not a direct physical property.
• Evaluate option C: Quark flavour is a categorical label in particle physics, not measurable with magnitude.
• Confirm option D: Spring constant quantifies stiffness in Hooke's law, measurable in N/m.

Why D is correct:

• Spring constant $k$ appears in Hooke's law $F = -kx$, representing restorative force per unit displacement with units newtons per meter.

Why the others are wrong:

• A: Metre is a SI unit for length, not the quantity.
• B: Percentage uncertainty is a relative error metric, not a fundamental physical property.
• C: Quark flavour is a discrete quantum number, lacking numerical magnitude and units.

Final answer: D