In the context of the input sentence, determine the relationship between 'Syriza' and 'Alexis Tsipras'.
Options: subsidiary, legal form, manufacturer, chairperson

ATHENS (Reuters) - Greek Prime Minister Alexis Tsipras contained a rebellion in his left-wing Syriza party to win parliamentary approval on Thursday for a second package of reforms required to start talks on a financial rescue deal.
chairperson