Watson and Crick’s original DNA model had 10 base pairs per turn. In other words, one complete twist of their proposed double helix contained ten paired sets of bases.
This geometry helped explain DNA’s dimensions: the model was approximately 20 angstroms wide, and each base pair was separated from the next by about 3.4 angstroms along the helix axis. One turn therefore measured roughly 34 angstroms. The paired bases occupied the interior, while the sugar-phosphate backbones ran along the outside.
Readers may encounter a nearby value of about 10.5 base pairs per turn. That figure describes the common B-DNA form more precisely under typical conditions, where the helical twist is not exactly an integer. The quiz’s answer refers specifically to Watson and Crick’s model, which is conventionally summarized as 10 base pairs per turn. DNA can also adopt A-DNA and Z-DNA conformations with different helical dimensions.