Answer : 1. "revolving"
Explanation :

The most appropriate option to fill in the blank is:

revolving

Explanation:

"Revolving" is the correct term to describe the motion of planets around the sun. Revolving implies orbiting or moving in a circular or elliptical path around a central point, which accurately describes the movement of planets around the sun.

"Circling" could also be considered, but "revolving" is more commonly used in astronomical contexts to describe the movement of celestial bodies around one another.

"Rotating" refers to the spinning motion of an object around its axis, which is not the correct term to describe the planets' motion around the sun.

"Rounding" is not the appropriate term to describe the motion of planets around the sun.