1.答案：It's based on the idea （that）all people are created equal,(which) is accepted by most people.
2.翻译：它建立在所有的人生来平等这个观点的基础之上,这一观点被大多数人所接受.
3.
1）用that引导同位语从句,解释中心词the idea的具体内容.
该从句不缺少任何成分,所以用that来引导.
2）用which来引导非限制性定语从句,来修饰先行词the idea.
which等同于the idea,在该非限制性定语从句中充当主语.