During negotiations, the Managing director remained obdurate.

Answer: Option A

Adamant : refusing to be persuaded or to change one's mind.
Tough : (of a substance or object) strong enough to withstand adverse conditions or rough handling.
Co-operative : involving mutual assistance in working towards a common goal.
Flexible : capable of bending easily without breaking.
Obdurate : stubbornly refusing to change one's opinion or course of action.

Synonym of Obdurate is Adamant