She is certain ________ her skills and will prove her worth in the competition.

• শূন্যস্থানে সঠিক উত্তর হবে - of.
- Complete sentence: She is certain of her skills and will prove her worth in the competition.

• Certain -  নিশ্চিত; সন্দেহাতীত।
- এর পর 'of' preposition-টি appropriately বসে। 
- Certain of - নিশ্চিত। 

Source: Bangla Academy Dictionary.