Which of the following dimensions represents a tetragonal unit cell?

The dimensions represents a tetragonal unit cell is $a=b\ne c;\alpha =\beta =\gamma ={90}^{\circ }$.

The tetragonal unit cell is characterised by an axis of fourfold symmetry around which a 90° rotation of the cell causes the atoms to coincide with their starting locations. Tetragonal crystals of various minerals, such zircon, as well as those of the elements boron and tin, can form.

Hence the option B is correct.