Correct Answer: Forever

Solution : The correct choice is the fourth option.

Explanation: "Eternity" refers to endless or infinite time, and "forever" directly aligns with this concept of infinite duration. Forever means an everlasting or infinite duration of time.

The meanings of the other options are as follows:

• A moment means a brief or short period of time.
• A flash means an extremely brief moment or sudden burst of light.
• Purity means the state or quality of being pure, free from contamination or immorality.

Therefore, the correct answer is forever.