How far do you agree that the need to set up a Common Market was the most important reason to a merger between Singapore and Malaya?
I agree to a certain extent. A Common Market allow Singapore's goods to be sold in Malaya without tariffs and could create industries and jobs.

The PAP government felt that a merger with Malaya would bring about rapid economic growth. With this merger, Singapore hoped that a Common Market would be set up to support Singapore's new industries. With this, goods can be bought and sold freely without being taxed. This would increase trade, expand industries and create more jobs.

But that is not the important reasons for merger. The PAP government wanted freedom from British rule so the best way is the merger. Tunku also wanted to solve communist problems as PAP was defeated in April 1961. He was concerned that the split would weaken the moderates within the PAP and allow the radicals to gain control of the party. Thus leads to forming a communist government in Singapore and use it as a base to spread communism in Malaya. In order to prevent that, Tunku agreed to merger.

In conclusion, i agree to a certain extent that setting up a Common Market was the reasons for merger as there are many other important reasons as well.