Answer: Sample D is appropriate for use in the Spectronic 20™.

D A B C E

Sample A has bubbles, sample C is in a cracked cuvette, and sample E is in a cuvette with fingerprints and dirt on the outside. All of these will adversely affect the way in which light travels through the sample. In sample B, the cuvette does not contain enough of the sample solution, so light will not pass through the sample.