**Answer:** Option **
A **

**Explanation:**

No. of observations = 6, Increased average = 1.5, Total increased = 6 × 1.5 = 9

Now, let the unit digit of the number whose digits have been interchanged = x and ten's digit of the number = y

Original number = (10y + x), Number after interchanging the digits = (10x + y)

So, (10x + y) – (10y + x) = 9 OR 9x – 9y = 9

**∴ **Required difference = x – y = 1.

**Smart Trick Method:**
For such questions you can refer to the table below

Difference in no. after interchanging the digits |
Difference in the digits of the original no. |

**9 (9 × 1)** |
**1** |

18 (9 × 2) |
2 |

27 (9 × 3) |
3 |

36 (9 × 4) |
4 |