Take context from the input text and decide on the accurate relationship between 'Syriza' and 'Alexis Tsipras' from the options provided.
Options: platform, operator, chairperson, legal form

ATHENS (Reuters) - Rebels angered by Greece's international bailout walked out of the leftist Syriza party on Friday, formalising a widely-expected split after leader Alexis Tsipras resigned as prime minister to pave the way for early elections.
chairperson