Why can A260 by A280 ratio of DNA sample be checked before restriction digest?

Correct answer: To estimate protein contamination

The absorbance ratio at 260 nm and 280 nm is used as a quick check of nucleic-acid purity. DNA absorbs strongly near 260 nm, while proteins, especially because of aromatic amino acids, contribute more strongly near 280 nm. A relatively pure DNA sample commonly has an A260/A280 value near 1.8, although the exact value can vary with conditions.

A lower or abnormal ratio may indicate protein or other contaminating material. Such impurities can interfere with restriction enzymes and make digestion unreliable, so checking the sample before digestion is useful. The ratio cannot tell the colour of an enzyme, count gel pores, or directly determine vector orientation. Therefore, option A is the appropriate answer.