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

ATHENS (Reuters) - The leftist Syriza party of former Prime Minister Alexis Tsipras will challenge Greece's creditors on unresolved aspects of the nation's bailout but implement unpopular reforms it has already agreed, it said on Sunday.
chairperson