Explanation:

- Serendipity refers to the occurrence of events by chance in a happy or beneficial way. It's about luck or a fortunate accident.

- Sibilance refers to a hissing sound, particularly in speech or literature. It's unrelated to serendipity.

- Godsend is something good that happens unexpectedly, similar to a blessing. It closely matches the concept of serendipity.

- Catastrophe means a sudden and widespread disaster. It's the opposite of something beneficial or fortunate.

- Surreptitious means kept secret, often because it would not be approved of. It's unrelated to luck or fortune.

Correct Answer: Godsend