Which characteristic causes Vega, Altair, and Regulus to be flattened at the poles and bulging at the equator?

The characteristic that causes Vega, Altair, and Regulus to be flattened at the poles and bulging at the equator is rapid rotation. This phenomenon is known as "oblateness," which occurs because the force exerted by rapid rotation counteracts gravity at the equator, causing the equatorial region to extend outward.

As a star spins quickly, centrifugal force acts more significantly on the material at the equator compared to the poles, leading to a bulging shape. Consequently, this results in a star that appears slightly flattened at the poles and wider at the equator. Rapid rotation is common in massive stars like Vega, Altair, and Regulus, which contributes to their distinctive shapes.

The other characteristics listed, such as high temperature, strong magnetic fields, and low density, do not directly relate to the mechanism of oblateness caused by rotation. High temperatures may influence a star's luminosity, and strong magnetic fields could affect stellar activity, but they do not lead to the flattening shape observed in quickly rotating stars. Low density can describe the physical state of a star but is not a factor that causes the flattening effect associated with rapid rotation.