Explanation:
> "You should make an ____ to improve the situation."

The sentence is talking about taking an action or trying to improve a situation, so we need a word that fits with the idea of exerting energy or trying to bring about a change.

Now, let's look at the options:

- A. effect — This refers to a result or outcome, not an action or effort. Incorrect.
- B. efficiency — This refers to how well something is done, not the act of trying to improve something. Incorrect.
- C. effectiveness — This refers to how well something works, but it doesn't fit in the context of making an attempt to improve. Incorrect.
- D. effort — This means the exertion of physical or mental energy to achieve something, which fits perfectly with the idea of trying to improve a situation. Correct.
- E. effectualness — This is similar to "effectiveness," but it's less commonly used in this context. Incorrect.