Shape of BF4−​ is:ATetrahedralBPyramidalCTrigonal planarDBent

Boron has 3 valence electrons, and each of the four fluorides contributes one electron to each covalent bond. The overall negative charge of the molecule contributes another electron, so overall we have 3 + 4 + 1 = 8 valence electrons which form 4 electron pairs.

The pairs arrange themselves in space to maximise the distance between them, thereby reducing the electronic repulsion between them to a minimum. The shapes for various numbers of electron pairs are:

2 - linear
3 - trigonal planar
4 - tetrahedral
5 - trigonal bipyramidal
6 - octahedral
7 and above - Google!

And hence BF4- would be tetrahedral.