What unusual property of water is responsible for frost wedging?

The property of water that it expands and becomes less dense as it freezes. The property is unusual because it  is not the case in other liquids. 
 Ice wedging is a form of mechanical weathering or physical weathering in which cracks in rock or other surfaces fill with water, freeze and expand, causing the cracks to enlarge and eventually break.

1. The fact that water requires more volume when it solidifies (freezes). Most liquids will turn into a solid when they freeze and in the process of freezing, take a smaller volume. But water is unusual in that when it freezes, it increases its volume. And during that expansion, it can exert a LOT of force. If water is in an enclosed space, it can exert about 1000 times atmospheric pressure against its surroundings.

Mass is the amount of matter in an object.  weight is the measurement of the pull of gravity on an object.  say for example, you travel to the moon which has a different gravitational field compared to the gravitational field of earth. (gravitational fields: earth = 1, moon = 0.165)  now when you are on the moon, the pull of gravity on you is different than on the earth. and as weight is the measurement of the pull of gravity on an object, that means yes, your weight can and will change on the moon.  however, on the moon, on earth or even on jupiter, the amount of mass inside of you remains the same. meaning your physical mass remains constant.  in conclusion it is possible for your weight to change as your mass remains constant. 

A.(nh4)(2co3)is the correct answer