Correct Answer:
C. of
The correct preposition to use with the adjective 'confident' when expressing certainty about something is 'of'. The phrase 'confident of' means having a strong belief or feeling of certainty about something or someone.
In the sentence, 'She was confident of her success' correctly conveys that she was certain she would succeed. Therefore, option 'C' is the accurate choice.
Distractor 'A' ('with') is incorrect; 'confident with' typically refers to comfort or skill in handling something. Distractor 'B' ('to') is not idiomatic in this context. Distractor 'D' ('for') is also grammatically incorrect here; 'confident for' is not a standard English idiom.