Which of the following pairs of compounds are enantiomers :

Enantiomers are non-superimposable mirror images. In Fischer projections, two structures are mirror images if every left and right group is swapped.
A structure of butane-2,3-diol in which both OH groups are on the same side (both right or both left) has a plane of symmetry through the middle. It is the meso form, which is optically inactive and has no enantiomer.
A structure in which the two OH groups are on opposite sides is chiral.
In the first pair, the left structure has OH on the right at C-2 and on the left at C-3 (chiral). The right structure has every group swapped: OH on the left at C-2 and on the right at C-3. They are mirror images.
Neither can be turned into the other by a $180^\circ$ rotation in the plane (which is the only allowed operation for Fischer projections), so they are non-superimposable mirror images, i.e. enantiomers.
In the other pairs, one member is a meso form (both OH on the same side), so those pairs are diastereomers, not enantiomers.
So the pair that are enantiomers is the first pair, with OH groups on opposite sides in both structures and every position swapped between them.