According to the law of conservation of energy, what will most likely happen in a closed system?
energy will be exchanged along with matter.
energy will be exchanged, but matter will not be exchanged.
energy will be created along with matter.
energy will be destroyed, but matter will not be destroyed.

I think the correct answer from the choices listed above is the second option.  According to the law of conservation of energy, in a closed system, energy will be exchanged, but matter will not be exchanged. Exchange of energy can happen on the sides of the system by heat transfer if it is not insulated. Also, in a closed system matter cannot be exchanged since matter cannot enter or leave the system.

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.