Question

Ritu was doubtful whether her answer was correct. (use: sure)

Answer 1

Main sentence -

Ritu was doubtful whether her answer was correct.

New sentence -

Ritu was not sure whether her answer was correct.

Explanation -

We will replace doubtful with sure and add "not" so as to make the meaning same of both sentences.

Answer 2

Ritu was not sure whether her answer was correct.