C.it
it在这里代替宾语从句to help those people in need
由于feel的实际宾语to help those people in need太长,为了避免头重脚轻,英语中常将其放在句尾,而用it 引导并放在前边,使句子平衡,符合英语习惯.
此句中,it作为形式宾语指代to help those people in need.
Our duty在句中充当宾语补足语
原句相当于We all feel to help those people in need our duty.（显得头重脚轻,不宜）
另举一例：He feels it his honour to serve the people.(他以为人民服务为荣.)