What is the relationship between 'Syriza' and 'Alexis Tsipras' in the context of the input sentence.
Options: currency, owned by, chairperson, headquarters location

Prime Minister Alexis Tsipras resigned last week after his acceptance of an 86 billion euro ($96 billion) rescue package for Greece's stricken economy triggered a major rebellion within his radical left Syriza party.
chairperson