For wide distribution of any plant, the means should have characteristics as follows :
1. It should be well protected from unfavourable conditions and physical attacks.
2. It should survive for longer period.
3. It should be a miniature form of plant, i.e., fertilized embryo should be present in it.
4. It should be light weight so that to survive from flood, strong wind.
5. It should not be vulnerable to pathogens.
6. It should have proper stored food to supply the seedling after germination.
Spore is a product of asexual reproduction. If spore gets favourable conditions, another partner should be available for reproduction.
Vegetative means are vulnerable and cannot survive for longer period.
Fruits are vulnerable to pathogens, environment and are not well protected from physical attack.
Seed is the embryonic plant which has a capacity of forming a new plant under favourable conditions. Also, seed is well protected from unfavourable natural conditions such as strong water current, strong wind flow etc. Seed fulfils all the above said conditions.
Therefore, the correct answer is option C.