Explanation:

- Envisage: To contemplate or conceive of a future possibility or plan. It involves visualizing or imagining something in your mind.

- Option 1 - Invent: To create or design something that has not existed before. This is more about coming up with innovative ideas or products.

- Option 2 - Repeat: To say or perform something again. It does not pertain to future planning or visualization.

- Option 3 - Imagine: To form a mental picture or concept of something. This closely aligns with the meaning of envisage, as both involve visualization.

- Option 4 - Purchase: To acquire something by paying for it. It is unrelated to the concept of visualizing or planning.

Option 3 - Imagine is the correct synonym. It emphasizes forming a mental image, similar to envisage.