Answer:
Common.
Step-by-step explanation:
The initial sentence reads-
Tsunamis are less prevalent in the Indian Ocean than in the Pacific.
Now, judging from the word choices given in the options, the word to change or replace will be "prevalent" with the word that has the same meaning and connotation. So, if that word "prevalent" has to be changed, it will be best replaced by "common". For, common is the of the same meaning of "prevalent". Besides, the other options "mild, severe, harmful" refers to the condition of the tsunami and not the time/ how often it occurs.
Thus, the final sentence will become-
Tsunamis are less common in the Indian Ocean than in the Pacific.