• Statement (I) is True: Native nucleic acids exhibit maximum UV absorbance at 260 nm ($A_{260}$) driven by the conjugated $\pi$-electron systems of the purine (adenine, guanine) and pyrimidine (cytosine, thymine) aromatic heterocyclic bases.
• Statement (II) is False: Thermal or alkaline denaturation of double-stranded DNA into single strands disrupts base stacking, resulting in hyperchromicity—an increase in absorbance by 30 to 40% at 260 nm, rather than a decrease.
• Conclusion: Statement (I) is true but Statement (II) is false.

Correct option: (3) Statement (I) is true but Statement (II) is false.