He adopted quite a diffident stand when it was a matter of son's career.

Answer: Option D

Confident : feeling or showing confidence in oneself or one's abilities or qualities.
Bold : (of a person, action, or idea) showing a willingness to take risks; confident and courageous.
Forward : in the direction that one is facing or travelling; towards the front.
Unassuming : not pretentious or arrogant; modest.
Diffident : modest or shy because of a lack of self-confidence.

Synonym of Diffident is Unassuming