Final answer:
A 300 bp long DNA molecule with 20 complete rotations is negatively supercoiled, as it is under-wound compared to the normal relaxed state of one turn per 10 base pairs.
Step-by-step explanation:
If a DNA molecule that is 300 base pairs (bp) long has 20 complete rotations, it is negatively supercoiled. In a relaxed DNA molecule, there is normally one turn of the helix per 10 base pairs. This standard is derived from the double helical structure of DNA, where there are approximately 10.5 base pairs per turn of the helix. Thus, a DNA segment with 300 base pairs would normally have about 30 turns to be in its relaxed state. Having only 20 turns indicates that it is under-wound, which corresponds to negative supercoiling.