What is the enigmatic stone tower at Naqsh-e Rostam, whose purpose is still debated, called?

• A Ka'ba-ye Zartosht ✓
• B Takht-e Soleyman
• C Borj-e Gonbad
• D Atashgah of Estakhr

The 'Cube of Zoroaster' is an Achaemenid tower facing the royal tombs; theories range from fire temple to archive.