Step 1: Picture the flow. A real liquid moving through a tube does not slide as one solid block. Different layers move at different speeds because the liquid has viscosity, which is internal friction between layers.
Step 2: Apply the no-slip condition. The very thin layer touching the wall of the tube sticks to it and stays almost still. This is the no-slip condition. So speed is least at the wall.
Step 3: Find where speed is largest. As we move away from the wall toward the centre line, each layer drags a bit less and moves faster. The fastest flow is right at the centre of the tube.
Step 4: Read the layer positions. In the given tube the three marked layers sit at different distances from the wall. The layer nearest the centre carries the highest speed, and the layer nearest the wall carries the lowest.
Step 5: Order the speeds. Following the layer arrangement shown, the speeds rank so that $u_3$ is fastest, then $u_2$, then $u_1$.
Step 6: State the order. The correct speed order is therefore as boxed. \[ \boxed{u_3 > u_2 > u_1} \]
