Why does WAF block HTTP OPTION method

web browser sometime sends http options request as safety measures for cross domain transaction, e.g. the webpage is opened from domain A but it also has javascript that sends http post request to domain B
http options request is sent to url in domain B.
https://aws.amazon.com/what-is/cross-origin-resource-sharing

if you're admin of domain B, then you need to allow http options request.