The word, ‘adventurous’ is an adjective which means daring/bold. It also means ‘willing to take risks or experiment with new ideas'. The word, 'fearful' means afraid/scared. It may be observed that the two words convey opposite meanings. Hence, 'fearful' is the antonym of 'adventurous' and option B is the correct answer.
The word, ‘fearless’ means ‘brave/bold'. It is a synonym of ‘adventurous’. So option A is incorrect.
The word, ‘dangerous’ means ‘likely to cause harm’. The word, ‘careless’ means ‘not giving enough attention’. As both the words 'dangerous' and 'careless' do not convey the same meaning as afraid/scared, options C and D are incorrect.