The correct option is B C>B>A
Basic strength∝1% s character
In A : Hybridisation is sp. % s character is 50%
In B : Hybridisation is sp2. % s character is 33.3%
In C : Hybridisation is sp3. % s character is 25%
More the s−character more is the electronegativity, and more stable the negative charge, so less will be the basic strength.
Correct order is :