total no of five digit numbers formed by using each of the digits 1, 2, 3, 4, and 5 exactly once =5!=120

no of five digit numbers formed by by using each of the digits 1, 2, 3, 4, and 5 exactly once and 1,2 are together in each of digits =4!*2!(here 2! because 1,2 can be arrranged in 2! ways) =48

now no of 5 digit numbers formed by using each of the digits 1, 2, 3, 4, and 5 exactly once and 1,2 are not together in each of digits =120-48=72

so the correct option is D