Answer is: 7. Lewis base.
Nitrogen trifluoride (NF₃) a Lewis base, because it can donate its lone pair of electrons (lone pair of electrons are in nitrogen atom).
Lewis base is any species that has a filled orbital containing an electron pair which is not involved in bonding but may form a dative bond with a Lewis acid.
Nitrogen-fluorine compound is a colorless, odorless and nonflammable gas.
