Eustress means good stress with the opposite being distress. So eustress would be any stress that is actually beneficial to your well being and has a positive effect.
From the options listed the ones that apply to eustress would be:
improves performance
maintains motivation
focuses energy
These are all good types of stress to have. Eustress would affect social relationships positively and would have positive emotional/mental effects but as these two options are not more specific they cannot be counted as correct.