What can be compared to estimate the size of fragments made by restriction enzymes?

Correct answer: DNA ladder

A DNA ladder, also called a molecular-size marker, contains DNA fragments of known lengths. During agarose-gel electrophoresis, DNA molecules move through the gel according to size: smaller fragments generally travel farther than larger fragments. The bands produced by restriction digestion are run beside the ladder, and their positions are compared with marker bands to estimate fragment sizes, usually in base pairs. Therefore option A is correct. Chlorophyll colour, soil particles and protein taste provide no calibrated DNA-length reference and cannot be used to interpret restriction fragments on a gel. The ladder is a comparison standard, not an enzyme or a digestion product.