Which of the following has a planar structure ?

In ${\mathrm{XeF}}_4$ the ‘Xe’ atom is ${\mathrm{sp}}^3{\mathrm{d}}^2$ hybridised, which contains two lone pairs and four bond pairs. 

Therefore the shape of the ${\mathrm{XeF}}_4$ molecule is square planar.

In ${\mathrm{NH}}_4^{+}$, central atom(N) is sp³ hybridized and the shape is tetrahedral

In SCl₄(1l.p+4b.p), Sulphur is sp³d hybridized and the shape is see-saw.

In ${\mathrm{BF}}_4^{-}$, Boron is sp³ hybridized and the shape is tetrahedral.