The nitrogen atom with least tendency to donate electron pair is

In (b), lone pair is not involved in resonance, $\therefore$ (b) is the best electron pair donor.

In (c), lone pair is involved in resonance with benzene ring, (c) is a weak electron pair donor.

In (a), lone pair is involved in resonance with two strong electron withdrawing -CO- groups on both sides, (a) is the weakest electron pair donor.