What was the main reason imperial nations built large naval forces? to initiate sea-based attacks on other nations to demonstrate their size and prowess to protect their economic interests to resist attacks on their harbors

The main reasons that imperial nations built large naval forces was to protect their economic interests in these colonies as well as their political interests. In addition, they also sought to demonstrate their size and prowess in their ability to exert military force around the globe. 

The answer is
to protect their economic interests