The right answer is C.
Note that in The Water Level vs Time graph, a constant level is represented by a line with slope=0 (for example, segment 2), when the level increases (when the water is running in the tub) the slope is positive (for example, segment 1) and when the level decreases (when Alison turns the water off) the slope is negative (for example, segment 5).
Now, if we want to prove this, let’s read again the statement in option C, dividing each section (see figure attached):
1. Alison turns on the water and allows it to run in the tub for a few minutes (as the water runs in the tube, the water level increases)
2. She then turns the water off while she runs to answer the front door. (when this happens the water level is constant)
3. Alison comes back and allows water to run into the tub for a couple more minutes (again the water level increases)
4. She takes a short bath (the water level is constant because she turned the water off)
5. Alison allows the water to completely drain from the tub (When this happens, the water level decreases)