According to article ii the constitution ,which body of government checks a legislative authority of congress ?

The article II of the constitution, vests the executive branch power in the president of United States. The executive branch has the authority to check on the various laws passed by the legislative branch. The domestic affairs and foreign policy are its two main responsibilities. 

The weakness of India was only that it was no united

The British were able to take control of India only because India was not united. The British signed treaties and made military and trading alliances with many of the independent states that made up India. The British were very effective at infiltrating these states and gradually taking control.