Answer: My boss, who was recently promoted again, is great at motivating her employees.
Explanation: A nonrestrictive clause is always surrounded by punctuation marks (commas, parentheses or dashes) to clearly define the information that could be removed from the sentence without altering its meaning. The dashes in options 1 and 3 combined would also make a correct sentence, but not on their own. In the last option, the second comma is wrong for two reasons: it is outside of the relative clause and it is separating the verb motivating from the object her employees.
