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.
