Take context from the input text and decide on the accurate relationship between 'Syriza' and 'Alexis Tsipras' from the options provided.
Options: brand, developer, member of, chairperson

Having reluctantly agreed to stringent terms for a new bailout deal, Greek Prime Minister Alexis Tsipras must face down a rebellion in his anti-austerity Syriza party to push sweeping pro-market reforms and spending cuts through parliament.
chairperson