Option C) self-fertilisation is prevented is the correct option. As, in homosporous vascular cryptogams like ferns, single type of spores are produced which give rise to haploid gametophyte on germination. The haploid gametophyte produces both male and female gametes but cross fertilization occurs because of the fact that archegonia (producing egg) and antheridia (producing antherozoids) mature at different times.As a result, of which self-fertilisation is prevented and cross fertilization occurs.

Antherozoids and eggs mature at different times in the prothallus of a vascular cryptogam. This has several implications for the plant's reproductive process.

Explanation:

1. Self-fertilization is prevented:
- Vascular cryptogams, such as ferns, typically exhibit an alternation of generations life cycle. This means they have a haploid gametophyte phase (prothallus) and a diploid sporophyte phase.
- The prothallus is the gametophyte stage where the antherozoids (sperm) and eggs (female gametes) are produced.
- In order to prevent self-fertilization and promote genetic diversity, the antherozoids and eggs mature at different times.
- If both the antherozoids and eggs matured simultaneously, there would be a higher chance of self-fertilization, resulting in reduced genetic variability.

2. High degree of sterility:
- Due to the asynchronous maturation of antherozoids and eggs, there is a time gap between their availability for fertilization.
- This time gap increases the likelihood of antherozoid loss through dispersal or degradation before the eggs become receptive.
- As a result, there is a higher degree of sterility in the prothallus, as successful fertilization requires the timely presence of both antherozoids and receptive eggs.

3. No change in the success rate of fertilization:
- The asynchronous maturation of antherozoids and eggs does not necessarily affect the success rate of fertilization.
- Although there is a higher degree of sterility, the remaining antherozoids that manage to reach the receptive eggs can still fertilize them successfully.
- Therefore, even though there may be a reduced number of successful fertilizations due to antherozoid loss, the success rate of the remaining antherozoids is not affected.

In conclusion, the asynchronous maturation of antherozoids and eggs in the prothallus of a vascular cryptogam prevents self-fertilization, increases sterility, but does not affect the success rate of fertilization for the remaining antherozoids.