Your answer would be C because the word candle in the sentence is plural, and it agrees with the word "were" because if you were referring to the past as of how long ago the candles were on the cake, C would be a good answer.
Example: Sixty candles were on the cake twelve minutes ago.
If you were to add a time period or specific date, it'd make sense as to when you were talking about how many candles were on the cake.
I hope I helped :)
