Chuck has a very detailed model of the solar system where each planet is made out of granite rock. since his little sister really liked the model, he spent two weeks in his dad's workshop building her a styrofoam copy. when he picked up the models to take them back inside the house, which physical property would he immediately notice upon picking them up?

He could be blindfolded and know which was his and which was his sister's. All he would need to do is pick them both up and if they were too big then pick them up one at a time. The lumber might make it harder to tell, but this is a question about physical properties.

So there is a change in mass which for the purpose of this question should be  quite different. His sister's ought to be much lighter than his. He would find it easier to pick up.

answer; i believe that the correct answer is (a); a large rock being weathered by wind and rain;

thats how its supposed to be