The correct answer is option 1, heating. Hyperchromicity refers to an increase in absorbance or intensity of UV light by a molecule. In the case of DNA, heating can cause it to denature, which means the two strands separate. This denaturation leads to an increase in the absorbance of UV light by DNA, resulting in hyperchromicity. When DNA strands separate, the bases are exposed and can absorb more UV light. This increase in absorbance is commonly observed at 260 nm and is used as a measure of DNA denaturation. Other options mentioned in the question, such as cooling, crystallizing, and replication, do not directly cause hyperchromicity in DNA. Instead, they are unrelated processes or phenomena.