In Pteridophytes, the dominant generation is

The correct answer is diploid.

To understand why this is the case, we can look at the alternation of generations in Pteridophytes (which include ferns and horsetails).
In plants, the life cycle alternates between two distinct multicellular phases:
1. The gametophyte generation, which is multicellular and haploid (containing one set of chromosomes, denoted as n). It produces gametes (sperm and egg cells) by mitosis.
2. The sporophyte generation, which is multicellular and diploid (containing two sets of chromosomes, denoted as 2n). It produces spores by meiosis.

Unlike bryophytes (like mosses and liverworts) where the haploid gametophyte is the dominant, photosynthetic, and independent phase, pteridophytes show a major evolutionary shift.
In pteridophytes, the dominant, main, and prominent plant body is the sporophyte. This sporophyte generation is independent, photosynthetic, and vascularized (differentiated into true roots, stem, and leaves).

Since the sporophyte is the dominant phase in Pteridophytes and its cells contain two complete sets of chromosomes, the dominant generation is diploid (2n).