Select the most appropriate option to fill in the blank.

The planets are ______ around the sun.

revolving

The correct option to fill in the blank is revolving.

The planets revolve around the sun in elliptical orbits. The word "revolve" means to move around something in a circle or an oval. It is the most appropriate word to use in this context, as it describes the motion of the planets around the sun.

The other options are not as appropriate:

• Circling means moving around something in a circle. It is a synonym of "revolving", but it is not as specific.
• Rotating means spinning around on an axis. It is not accurate to use "rotating" in this context, as the planets are not spinning around on their own axes.
• Rounding means making something round or curving something around something else. It is not accurate to use "rounding" in this context, as the planets are not rounding the sun.