The correct answer is option-1, a law passed by the parliament.

Establishing a common High Court for two or more states requires a law to be passed by the parliament. This means that the Indian parliament, which is the supreme legislative body in the country, has the authority to create a common High Court for two or more states.

Option-2, an order of the Supreme Court of India, is not the correct answer. The Supreme Court of India is the highest judicial authority in the country and its role is to interpret the Constitution and uphold the law. It does not have the power to establish a common High Court for two or more states.

Option-3, an order of the President of India, is also not the correct answer. The President of India is the head of state and holds a ceremonial position. While the President does have certain powers related to the appointment of judges, the establishment of a common High Court requires a law passed by the parliament.

Option-4, an amendment to the Constitution of India, is also not the correct answer. An amendment to the Constitution would be required for significant changes to the structure or functioning of the judiciary, but establishing a common High Court does not necessarily require a constitutional amendment.