In the context of the input sentence, determine the relationship between 'Syriza' and 'Alexis Tsipras'.
Options: brand, member of, chairperson, 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