What was the main reason imperial nations built large naval forces? to initiate sea-based attacks on other nations to support nearby merchants to protect their economic interests to resist attacks on their harbors

Imperial countries built large navies to protect their economies interests.

Explanation:

Imperial countries built large naval forces to protect their economic interests. As the sea is the route of all the trade and export and import taking place the hub nations need to increase the security on the harbours from the other nations who might attack.

There is a need to built a large naval force which can fight in the emergency and when the economy is in danger.