Is it correct to say
What was your happiest moment in 2015?
What is your happiest moment in 2015?
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Normally we would use was, because normally we think of 2015 as complete and belonging to the past, and therefore we would use the past tense.
Using is is possible if 2015 is thought of as "present." For example, if the person you are taking to
1 thinks it is still 2015
2 too drunk to know that 2015 is over
3 has time travelled back to 2015
4 has been hypnotized to believe it is 2015
5 is reenacting 2015
6 is always in 2015
you could ask her What is your happiest moment in 2015?