Among the given, $B F_{3}$ is an electron deficient species, so have a capacity of electron accepting instead of donating. That's why it is least likely to act as a Lewis base. It is a Lewis acid.
(આપેલ : $\left.pK _{ a }\left( CH _{3} COOH \right)=4.76\right)$
$\log 2=0.30$
$\log 3=0.48$
$\log 5=0.69$
$\log 7=0.84$
$\log 11=1.04$