I have a doubt about the answer.

There is no doubt that she is the best candidate for the job.

I am doubtful about his intentions.

The lawyer doubted his innocence.

There is no doubt that she is the best candidate for the job.

I have doubts about the success of their plan.

John is a doubting Thomas.

Without doubt, she is the best singer in the world.

His actions leave no doubt about his intentions.

When in doubt, ask for help.