Reduction division in pteridophytes occurs in

The correct option is "spores are formed".

To understand why reduction division (meiosis) occurs during spore formation in pteridophytes, let us look at their life cycle step-by-step:

1. Dominant Sporophyte Phase:
The main plant body of a pteridophyte (like ferns) is a sporophyte. This sporophyte stage is diploid, meaning its cells contain two sets of chromosomes ($2n$).

2. Formation of Spores (Sporogenesis):
The sporophyte produces special structures called sporangia. Inside these sporangia, spore mother cells (which are diploid, $2n$) undergo cell division to produce haploid spores ($n$). For the chromosome number to be halved from diploid ($2n$) to haploid ($n$), a reduction division is required. This reduction division is called meiosis.

3. Development of Prothallus and Gametes:
Once the haploid spores ($n$) germinate, they grow into a multicellular, free-living haploid gametophyte structure called the prothallus. Since the prothallus is already haploid ($n$), the sex organs formed on it and the gametes (antherozoids and egg cells) they produce are formed via mitotic division (equational division), not reduction division.

Therefore, reduction division (meiosis) in pteridophytes occurs strictly when spores are formed from the diploid spore mother cells.