[quote user="K VIJAYA RAM BHARADVAJ"]Sir, but my doubt is about two different formulas.[/quote]
Because you did not look at the ADC12 block diagram.
One formula is for clock of ADC12 core, other for ADC12 peripheral block input clock, accounts divider to calculate ADC12 core clock. ADC peripheral have ADC core, divider and clock input selector. So both formulas are valid actually. Look the funny block diagram of ADC12!!!
ADC core takes 13 clocks for conversion, so use frequency of ADC core, after divider, using formula tconvert=13*(1/ADC12CLK)
[quote user="K VIJAYA RAM BHARADVAJ"]1. t convert=13*4/32768=0.001586sec[/quote]
This clock will be too slow - out of spec. Check datasheet for allowed ADC12 clock frequency range.