The cell membrane around a cell forms a barrier that protects and regulates the cell. which best describes those that can pass through?

Ok I think its small polar molecules but I could be wrong. 

Ifound this on google “according to the findings, a foramen magnum positioned toward the base of the skull is found not only in humans, but in other habitually bipedal mammals as well.” i hope this you for what your looking for

Where are the following choices