Final answer:
Doug ceased to offer new ideas because his coworkers rejected his initial suggestion. Rejection is the action that would logically cause someone to stop contributing ideas in a group setting.
Step-by-step explanation:
Doug stopped offering new ideas after the other workers rejected his first suggestion. In this sentence, the correct word to complete the blank is rejected. It implies that his coworkers did not accept his initial idea, which subsequently influenced his decision to refrain from suggesting further ideas. Had the coworkers embraced, implemented, or improved his idea, it would have likely encouraged him to continue contributing more ideas.