Which of the following statements is true?

All of the sun's radiation is reflected back into space by the Earth's atmosphere.
Air is a good insulator because the molecules in gas are very far apart.
Work can only be done by potential energy.
Total mechanical energy is calculated by subtracting kinetic energy from potential energy.

Air is a good insulator because the molecules are far apart. This reduces mobility of air molecules hence conduction is difficult.

Explanation:

For other statements :

All the suns radiations are not reflected back to space because the ozone layer of the earth's atmosphere which regulates the intensity of radiations from the sun, it is transparent and porous hence heat and light rays from sun enter earth.Work is only done by potential energy when the body is under free fall and experiencing gravitational force, but for a body moving without influence of gravity, work is done by the force applied and the distance moved by the body. This force is called a conservative force.Total mechanical energy is given by

= Kinetic energy + Potential energy