Answer:
both the hydrophobic effect and van der Waals interactions.
Step-by-step explanation:
Hydrophobic interactions refer to the interaction of non-polar groups with each other in the watery medium. Van der Waals interactions are the weak intermolecular forces formed when one molecule induces polarization in the other.
A DNA double helix has the base pair stacked on the top of one another. The adjacent base pairs are separated from each other by a distance of 3.4 angstroms. The atoms of each base pair for van der Waals interactions with each other since they are separated from each other by van der Waal contact distance.
Also, the hydrophobic surfaces of the bases make hydrophobic interactions with each other to stay away from the water. Therefore, Van der Waal interactions and hydrophobic effects both contribute to base stacking.