Step 1: Understand the issue.
The Constitution creates the Supreme Court but does not fix its number of judges forever.
Step 2: Find the provision.
Article 124(1) says the Court has the Chief Justice and as many other judges as Parliament may by law decide.
Step 3: See how it works in practice.
Parliament passes the Supreme Court (Number of Judges) Act to raise the strength when the workload grows. In 2019 it raised the figure from 31 to 34.
Step 4: Rule out the others.
The Prime Minister cannot change it alone, the President only appoints judges, and the Court itself cannot change its own sanctioned strength by law.
Answer: The Parliament of India (Option 1).