“choosing is refusing” means individuals face an opportunity cost with each decision they make

This is true because when you choose one thing, you refuse the other thing, so choosing is indeed refusing, and it fits the definition of opportunity costs.

Ifound the exercise on the internet. attached is the list mentioned above.in order for a compound to be considered optically active it can't have any axis or plan of symmetry. the molecule  shouldn't also  be superimposable on its mirror image.with such considerations, the compounds that are optically active are: -third compound on the first image is optically active; -second and third compound from the second image are optically actives.