What is the relationship between Optum and UnitedHealth Group 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.

He joined the government in 2014, after working as an executive vice president at UnitedHealth Group Inc.'s Optum unit, which helped fix the main Obamacare website, healthcare.gov.

Answer: parent_organization