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

Greek Prime Minister Alexis Tsipras is also under pressure from many in his radical left Syriza party who say the new accord will pile further austerity on a weakened economy and goes against the party's campaign pledges.
chairperson