Which of these was a major Iranian city in Central Asia?

Merv was a major Iranian city in Central Asia, on the historical Silk Road, located near today's Mary in Turkmenistan. Several cities have existed on this site, which is significant for the interchange of culture and politics at a site of major strategic value.

Human settlements on the site of Merv existed from the 3rd millennium BC until the 18th century AD. It changed hands repeatedly throughout its history. Under the Achaemenid Empire, it was the centre of the satrapy of Margiana. It was subsequently ruled by the Arabs, Turks and the Safavid Persians. During the 12th and 13th centuries it became one of the largest cities in the world, having a population of as many as 500,000. In 1221 the city opened its gates to an invading Mongol horde; the resulting destruction of the city proved devastating. Historical accounts contend that the entire population (including refugees) of a million people were killed.