What is the name of the missing letter in the Greek numeral system representing the number 6, originally named 'wau' and later replaced by the ligature stigma?

• A digamma ✓
• B koppa
• C sampi
• D qoppa

In the Greek alphabetic numeral system, digamma (ϝ) represented the number 6; though the letter became obsolete phonetically, its use as a numeral persisted through the stigma ligature (Ϻ).