What significant change did Ataturk make to the Turkish calendar as part of his modernization efforts in the first half of the 20th century?

Replaced it with the Western (Gregorian) calendar

The correct answer is Option 2- Replaced it with the Western (Gregorian) calendar

Ataturk replaced the traditional Turkish calendar with the Western (Gregorian) calendar as part of his modernization efforts.

Secularism practised in Turkey in the first half of the twentieth century: 

This secularism was not about principled distance from organised religion, instead it involved, active intervention in and suppression of, religion. This version of secularism was propounded and practised by Mustafa Kemal Ataturk. He came to power after the First World War. He was determined to put an end to the institution of Khalifa in the public life of Turkey. Ataturk was convinced that only a clear break with traditional thinking and expressions could elevate Turkey from the sorry state it was in. He set out in an aggressive manner to modernise and secularise Turkey. Ataturk changed his own name from Mustafa Kemal Pasha to Kemal Ataturk (Ataturk translates as Father of the Turks). The Fez, a traditional cap worn by Muslims, was banned by the Hat Law. Western clothing was encouraged for men and women. The Western (Gregorian) calendar replaced the traditional Turkish calendar. In 1928, the new Turkish alphabet (in a modified Latin form) was adopted.