The correct answer is 'could have done'.

Explanation:

When we use the phrase "could have done," we are referring to a past action or decision that could have been done differently. It indicates a missed opportunity or a different choice that could have been made.

Example:
Imagine you are reflecting on a situation in the past where you made a mistake or could have acted differently. You might say:

"I could have done things differently."

In this sentence, "could have done" suggests that you had the ability or opportunity to make a different choice or take a different action at that time.

Usage:
"Could have done" is used to express regret, reflect on past actions, or consider alternative possibilities. It emphasizes the idea that something different was possible in the past but was not ultimately done.

Example:
Here are a few examples of how "could have done" is used in different contexts:

1. Regret:
- "I could have studied harder for the exam."
- "I could have been more supportive during their difficult time."

2. Reflection on past actions:
- "I could have handled the situation better."
- "I could have chosen a different career path."

3. Alternative possibilities:
- "We could have taken a different route to avoid traffic."
- "She could have chosen a different restaurant for dinner."

In all these examples, "could have done" acknowledges that a different choice or action was possible in the past but was not pursued.

Conclusion:

The correct answer "could have done" is used to discuss past actions or decisions that could have been done differently. It expresses regret, reflection, or alternative possibilities.