Answer: From B to D (top right corner)
Step-by-step explanation:
As the hint suggests, the velocity corresponds to the slope of the tangent line. As the curve goes downhill, when moving left to right, the tangent slopes will be negative.
In short, the downhill motion indicates we have negative velocity. This occurs on the portion from B to D on the curve. You can think of it like a rollercoaster. When going from D to F is when the curve goes uphill again and we have positive velocity.
Positive velocity = object is speeding up
Negative velocity = object is slowing down