Answer:
Venus < Earth < Mars < Saturn
Step-by-step explanation:
Orbital period is defined as the time taken by an astronomical bodies to complete one revolution around another astronomical body.
Kepler's law gives the relationship between the orbital period and the semi major axis of the orbit. Mathematically, it can be written as :

The planet that is near to sun will have smaller orbital period. So, Mercury have least orbital period out of given options while Saturn have greatest orbital period.
Hence, the correct option is (a) " Venus < Earth < Mars < Saturn ".