Answer:
The fish swimming upstream is going against the current and thus requires energy.
Step-by-step explanation:
Active transport is where energy (ATP) is required to move substances against the concentration gradient. Normally in simple diffusion, substances would go from a high concentration to a low concentration which does not require energy. However, if you go from low concentration to high concentration, you can't do that without energy. The process that allows this is active transport.
A is correct because going upstream is like going against the stream. Naturally, it's easier to go with the stream. But if you are going against the stream, it would require energy and it would be harder.
B is incorrect because the slope of going downhill kind of allows the child to go down without energy. This would be compared to active transport if the child was going uphill.
C and D are both incorrect because neither the sailboat or car are going against anything.