Take context from the input text and decide on the accurate relationship between 'Syriza' and 'Alexis Tsipras' from the options provided.
Options: operator, product/material produced, parent organization, chairperson

ATHENS, July 23 Greek Prime Minister Alexis Tsipras faced down a revolt by rebels in his leftists Syriza party to win parliament's backing on Thursday for a second package of reforms required to start talks on a financial rescue deal.
chairperson