The term dominant generation refers to

The dominant generation is haploid, so that the gametophyte comprises what we think of as the main plant. The opposite is true for tracheophytes (vascular plants), in which the diploid generation is dominant and the sporophyte comprises the main plant.

The term dominant generation refers to Bryophytes.

if your own plato the answer is b. the fern transitioned from sporophyte generation to gametophyte generation.