slope of velocity time graph is zero always the motion is
1 non accelerated
2 accelerated
3 retarded
4 both 1 and 3

Dear Student,

The slope of velocity-time graph gives us acceleration. 
As it is mentioned that the slope is zero, there will be no acceleration.

We can also visualize it as follows,
If the slope of velocity time graph is 0 then it is likely to be as follows: 

In this case, we can also calculate the acceleration and find it to be zero.

Thus the motion is non accelerated, the correct option is (1)

Regards