Armed with a newfound belief in the

Solution

The sentence tells us how psychologists were armed with a newfound (recently discovered) belief in the possibilities of behavioural conditioning, and using this belief, they attacked schools, homes and workplaces to advocate for sound conditions and stop adverse phenomena. DISSENTED means disagreed; DESCENDED would have been more apt here. MUSTER means to assemble; FOSTER means to encourage the development of something, and would be more apt here. DELEGATE means entrust a task to someone else; MITIGATE would have been more apt here. Since 3 words are inappropriate, (d) is the right answer.