ODD ' numbers. Once in a year We get only one New year for every year. 1 is an Odd Number. Twice in a month We get 4 New weeks in every month. In that we have 1,3 are odd weeks. Four times in a week We get 7 New days in Every Week. In that 1,3,5,7 are the ODD Numbers. Six times in a day We get 12 New hours every day. In that 1,3,5,7,9,11 are the ODD Numbers.
The answer is letter - "F".

In a year - February.
In a month - Four weeks - First week and Fourth week. 
In a week - Seven days - First day, Fourth day and FiFth day.
In a day - 24 hours - Four, Five, Fourteen, FiFteen, Twenty Four