Answer : The correct option is (a).
Explanation :
Velocity : It is defined as the rate of change of position with respect to the time. It is a vector quantity.
For the velocity, both magnitude and direction are needed to define it.
The formula of velocity is,

In option (a), both magnitude and direction are given. But in another options, directions and magnitude are not given.
According to the definition of velocity, the direction and magnitude are the important parameters without these parameters we can not described velocity.
So, the correct option is (a) the car traveled 70 mph north to Atlanta on Interstate 75.