The correct answer is option 3, Nepal. In September 2015, Nepal adopted its first democratic Constitution.

Option 1, Bhutan, is not the correct answer because Bhutan adopted its first democratic Constitution in 2008, not in September 2015.

Option 2, Myanmar, is also not the correct answer because Myanmar transitioned to a democracy in 2011, but its current Constitution was adopted in 2008, not in September 2015.

Option 4, Singapore, is not the correct answer because Singapore is not known for adopting a democratic Constitution in September 2015 or at any other time.

Therefore, the correct answer is option 3, Nepal, as it is the only country among the options that adopted its first democratic Constitution in September 2015.