**Explanation:**

To find the answer to this question, we need to understand how to determine the greatest 5-digit number and what happens when we add 1 to it.

**The greatest 5-digit number:**
The greatest 5-digit number is a number with 5 digits, where the leftmost digit is the largest possible digit (9) and the remaining digits can be any digit from 0 to 9. This means the greatest 5-digit number is 99999.

**Adding 1 to the greatest 5-digit number:**
When we add 1 to the greatest 5-digit number (99999), we get 99999 + 1 = 100000. This number has 6 digits, and it is the smallest 6-digit number.

Now, let's analyze the answer choices one by one to understand why the correct answer is option 'D'.

**a) Smallest 5-digit number:**
A 5-digit number has 5 digits, and the leftmost digit can be any digit from 1 to 9. The smallest 5-digit number is 10000. Adding 1 to this number gives us 10000 + 1 = 10001, which is a 5-digit number. Therefore, the correct answer is not option 'A'.

**b) Smallest 4-digit number:**
A 4-digit number has 4 digits, and the leftmost digit can be any digit from 1 to 9. The smallest 4-digit number is 1000. Adding 1 to this number gives us 1000 + 1 = 1001, which is a 4-digit number. Therefore, the correct answer is not option 'B'.

**c) Smallest 3-digit number:**
A 3-digit number has 3 digits, and the leftmost digit can be any digit from 1 to 9. The smallest 3-digit number is 100. Adding 1 to this number gives us 100 + 1 = 101, which is a 3-digit number. Therefore, the correct answer is not option 'C'.

**d) Smallest 6-digit number:**
A 6-digit number has 6 digits, and the leftmost digit is always 1. The smallest 6-digit number is 100000. Adding 1 to this number gives us 100000 + 1 = 100001, which is a 6-digit number. Therefore, the correct answer is option 'D'.

In conclusion, when we add 1 to the greatest 5-digit number, we get the smallest 6-digit number.

Because the greatest five digit is 99999 and when we add1 it become 100000