What is the relationship between Boeing 737 and Southwest Airlines in the context of the input sentence. Choose an answer from: product/material produced; manufacturer; distributed by; industry; position held; original broadcaster; owned by; founded by; distribution format; headquarters location; stock exchange; currency; parent organization; chief executive officer; director/manager; owner of; operator; member of; employer; chairperson; platform; subsidiary; legal form; publisher; developer; brand; business division; location of formation; creator.

WASHINGTON Aug 17 The U.S. Federal Aviation Administration (FAA) on Monday proposed a $325,000 penalty against Southwest Airlines Co for allegedly operating a Boeing 737 that was not in compliance with federal regulations.
operator