Utilize the input text as a context reference, choose the right relationship between 'Syriza' and 'Alexis Tsipras' from the options.
Options: brand, chairperson, member of, developer

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