If you pass this difficult examination, it will be a feather in your cap.
Here, the underlined part refers-

• If you pass this difficult examination, it will be a feather in your cap.
- Here, the underlined part refers to- খ) you will feel proud of it

• A feather in your cap (Idiom)
- English Meaning: an achievement to be proud of; something you have done that you should be proud of.
- Bangla Meaning: গৌরবনিদর্শন; গর্ব করার মতো অর্জন।

• More examples:
- This'll be a feather in her cap, right enough, a princess named after her.
- It will be quite a feather in his cap if Cambridge win today

Source:
1. Cambridge Dictionary.
2. Longman Dictionary.