Option 1: Strengthen -> This is a synonym of bolster, not an antonym.
Option 2: Weaken -> This is the opposite of bolster, meaning to make less strong or reduce support.
Option 3: Reinforce -> This is also a synonym of bolster, meaning to strengthen or support.
Option 4: Restrain -> This means to hold back or limit, but it's not the direct opposite of bolster.
Hence, Option 2: Weaken -> Bolster means to support, strengthen, or prop up something, so its antonym would be to weaken or undermine -> correct