Super-absorbent polymers are useful in fuel filters because they remove harmful moisture before it reaches the engine. These polymers have the ability to absorb and retain water or any other liquid, making them effective in removing water from fuel. By absorbing the moisture, super-absorbent polymers help prevent water-related issues such as corrosion, fuel degradation, and engine damage. So, the correct answer is d) they remove harmful moisture before it reaches the engine.