This word means the same as the given italicized word. Hence option D is correct.
Option A: 'Confident' means 'feeling or showing certainty about something'.
Option B: 'Bold' means 'showing a willingness to take risks'.
Option C: 'Forward' means 'confident and honest in a way that might seem rude'.
These words do not mean the same as the given word 'diffident'.
So, options A, B and C are wrong.