Distinguishing physical quantities from units and dimensionless ratios

Steps:

• Define physical quantities as measurable properties with dimensions (e.g., mass, time, charge).
• Examine each pair: both must be quantities, not units or dimensionless.
• Identify mismatches: units like ampere or pascal measure quantities; dimensionless like efficiency or strain are ratios without units.
• Confirm D: both have dimensions (time for period, energy per charge for potential difference).

Why D is correct:

• Period is time (dimension T), potential difference is electric potential (dimension ML²T⁻³A⁻¹), per SI base quantities.

Why the others are wrong:

• A: Ampere is the SI unit for current, not a quantity itself.
• B: Efficiency is a dimensionless ratio (work output/input), not a physical quantity.
• C: Pascal is the SI unit for pressure; strain is a dimensionless ratio (change/original length).

Final answer: D