If the slope of velocity-time graph gradually decreases, then the body is said to be moving with:

Explanation

On a velocity-time graph, the slope represents acceleration. A gradually decreasing slope indicates that the acceleration is negative, meaning the object is slowing down or experiencing deceleration. This change in velocity over time is what defines negative acceleration. On the other hand, options like positive acceleration and zero acceleration would imply either an increasing slope or a horizontal line, respectively, neither of which are consistent with a decreasing slope. Uniform velocity would show a constant, unchanging slope, also inconsistent with the scenario described.