Select the unitless quantities: a, absorbance c, solute concentration i/i0 (ratio of emergent to incident intensity) b, pathlength

Absorbance is unitless by definition, this is the fraction of light absorbed.

Solute concentration will have mol/L units, so it is not unitless.

i/i0 (ratio of emergent to incident intensity) is similar to absorbance, and the units will cancel out, making this unitless.

Path length will have a unit in millimeters or centimeters, so it is not unitless.

Therefore the unitless quantitites are absorbance and intensity ratio.