What linguistic phenomenon led to a prolonged 20th-century controversy between proponents of the archaic and vernacular forms of the language?

• A diglossia ✓
• B apocope
• C metathesis
• D synizesis

Diglossia (the coexistence of a 'high' formal language – Katharevousa – with a 'low' everyday language – Demotic) was at the centre of the Greek language question for most of the 19th and 20th centuries.