Question

floods can harm houses built near some rivers. when these houses are built high on posts, flood water safely flows under them. then, there is a reduction in the harm to homes. to create a building a decrease or lessening an increase or rise to make dangerous

Explanation:

The word "reduction" in the sentence "there is a reduction in the harm to homes" means a decrease or lessening of the harm. So we match it with the option "a decrease or lessening".

Answer:

a decrease or lessening