The correct option is
B Safeguard
The word 'imperil' refers to putting something at risk of being harmed, injured or destroyed.
Option A, safeguard refers to protecting something from harm or damage with an appropriate measure.
Option B, construct refers to the act of building or making something.
Option C, create refers to causing something to happen as a result of one's actions.
Option D, brighten refers to lightening up of something.
Thus, according to the meaning, the word 'safeguard' is the most opposite in meaning from the word 'imperil'.