The correct answer to this question is:
Europeans viewed the spread of Islam as a threat, and actively worked to stop it.
Explanation:
One of the main threats viewed from outside religions were that the people who had Muslim faith believed that those who were not Muslim should be killed, therefore the people who were not Muslim may have taken this as a threat. There are of course other reasons as well as some people may had taken this as a threat, but this is one of the main reasons.
(I took the test, this is correct)