If the acceleration of a body is zero then the slope of its v-t graph:

Explanation

The slope of a velocity-time (v-t) graph represents acceleration. If the acceleration of a body is zero, it means the velocity is constant, leading to a horizontal line on the graph, parallel to the time axis. Option A is correct as it correctly describes this scenario. Option B is incorrect because a line perpendicular to the time axis is not possible in this context. Option C and Option D both describe scenarios where there is a change in velocity, indicating the presence of acceleration, which contradicts the condition of zero acceleration.