The position-time (𝑥−𝑡) graph for positive acceleration is
 

We know slope of the displacement time graph is velocity.

Here, among the displacement time graph, (3) and (4) has constant slope. 

So, these velocities are constant. 

In graph (2), slope of the graph decreasing with time indicates that velocity is decreasing. Hence, moving with retardation.

In graph (1), slope is increasing with time hence, velocity is increasing, that means, it has acceleration.

So, graph (1) indicates the positive acceleration.