one

Any one of the boys.  The big one looks good.  I want the green one.  Every one of the bank’s employees.  A good driver is one who drives carefully.

She offered him an apple and an orange; he took one and left the other.

One’s guilt may trouble one, but it is best not to let oneself be troubled by things which cannot be changed.  One shouldn’t be too quick to judge.

"driver", noun: one who drives.