Physical quantities have measurable magnitude and SI units

Steps:

• Define physical quantity as a property with magnitude and units in the SI system.
• Check atomic number: it's a dimensionless integer counting protons.
• Check efficiency: it's a ratio of outputs to inputs, unitless.
• Check number density of charge carriers: it's carriers per unit volume, with units m^{-3}.
• Check strain: it's a ratio of deformation to original dimension, dimensionless.

Why C is correct:

• Number density of charge carriers is defined as n = N/V (carriers per volume), with SI unit m^{-3}, making it a measurable physical quantity in solid-state physics.

Why the others are wrong:

• A: Atomic number is a pure, unitless count used in chemistry, not measurable like length or mass.
• B: Efficiency is a dimensionless percentage (e.g., η = work output/work input), not a fundamental physical quantity.
• D: Strain is ε = ΔL/L, a unitless ratio in mechanics, classifying it as dimensionless.

Final answer: C