It's "reduction in the risk" is the right one.
Following is from wordreference forum about the use of right preposition:
1 a slight reduction in the price of oil
2 the reduction of interest rates
3 substantial reductions on children's clothes
These varying prepositions after certain verbs are always tricky even for native speakers. In your first sentence, "in" is most often heard before "price" though "of" is also possible. In your second sentence,
...
Following is from wordreference forum about the use of right preposition:
1 a slight reduction in the price of oil
2 the reduction of interest rates
3 substantial reductions on children's clothes
These varying prepositions after certain verbs are always tricky even for native speakers. In your first sentence, "in" is most often heard before "price" though "of" is also possible. In your second sentence,
...
