They were disspointed to see the armed guards.It ......... them from doing anything disruptive

From the sentence, it can be understood that the presence of armed guards stopped the subject of the sentence from doing anything (which they wanted to do presumably). 

Inspired, and encouraged would suggest that the subjects went ahead and did the disruptive things, which is opposite to what the sentence suggests. 

Irritated would also not fit since "It irritated them from doing anything" would not b grammatically correct. 

Prevented would best fit, showing that the presence of the armed guards prevented anything disruptive from happening. 

Therefore, Option B is the correct answer.