This is because the other variables in the other choices are relative to different observers and hence are not good to use in an experiment because there will be a lot of inherent bias by the person conducting the experiment For example how one person views how strong or beautiful the maple or oak looks may be different from how another person perceives the same.
However, weight is an absolute SI unit and does not vary from experiment to experiment. It is, therefore, a non-biased variable to use in the experiment and determine which wood between oak and maple can hold more weight.
