For retarded motion, the slope of the velocity-time graph is:

A body's velocity is defined as the rate at which its displacement changes. The magnitude and direction of velocity are both vector quantities in two dimensions.

Given 

Slope $=\frac{\Delta v}{\Delta t}$

The slope of a velocity-time graph represents the acceleration, which is defined as the rate of change in velocity.

For retarded motion, the final velocity of the object is less than the initial velocity of the object therefore, the slope of the velocity-time graph is negative. As a result, option 3 is right.