Let us assume the tens and the unit digits of the required number be x and y respectively

∴ Required number = (10x + y)

According to the given condition in the question, we have

x + y = 15 (i)

By reversing the digits, we obtain the number = (10y + x)

∴ (10y + x) = (10x + y) + 9

10y + x – 10x – y = 9

9y – 9x = 9

y – x = 1 (ii)

Now, on adding (i) and (ii) we get:

2y = 16

Putting the value of y in (i), we get:

x + 8 = 15

x = 15 – 8

x = 7

∴ Required number = (10x + y)

= 10 × 7 + 8

= 70 + 8

= 78

Hence, option D is correct