Sensory imagery, because its giving you a picture of what they're doing
The answer is A because "stolen from a high-security museum" is a sub-clause.
Yes, the boys are fearing the unknown rather than the beast.
Also no, because they are not fully sure the beast even exists.
The beast is depicted different to everyone
Wooden door vs glass door
Both are doors, serve the same function
Wooden is stronger
Glass you can see through