Answer:
BF_3 is a Lewis acid
Step-by-step explanation:
Lewis Acid is a type of molecule that takes an electron pair from an electrophile and has empty orbitals. In the choices given, BF_3 is a strong electrophile and has an empty orbital that can accept a lone pair of electrons. It is therefore a Lewis acid. The other three OH- H_3O NF_3 are Lewis bases because they have a single pair of electrons.