The correct option is A incautious
The underlined word 'tactful' means 'having skill and sensitivity in dealing with difficult issues'. It refers to a person as being thoughtful.
Option A: 'Incautious' means 'not being cautious to potential problems and risks'. It is the direct opposite of the given word. Hence A is correct.
Option B: 'Discreet' means 'careful and cautious'.
Option C: 'Strict' means 'stern or severe with rules and regulations'.
Option D: 'Disciplined' means 'showing a controlled behaviour or manner'.
These do not express the opposite meaning. Hence B, C and D are incorrect.