**Greatest 5-Digit Number Explanation**

To determine the greatest 5-digit number, we need to understand the concept of place value.

**Place Value:**

In our number system, every digit has a place value based on its position in a number. The place values start from right to left as units, tens, hundreds, thousands, ten thousands, and so on.

**Greatest 5-Digit Number:**

A 5-digit number has a digit in the ten thousands place, thousands place, hundreds place, tens place, and ones place.

In the given number, 99,999, the digit in the ten thousands place is 9. To make the number the greatest 5-digit number, we need to make the digit in the ten thousands place the largest possible.

**Options Explanation:**

a) 5: This option is incorrect because the digit 5 cannot be placed in the ten thousands place to make the number the greatest 5-digit number.

b) 4: This option is incorrect because the digit 4 cannot be placed in the ten thousands place to make the number the greatest 5-digit number.

c) 3: This option is incorrect because the digit 3 cannot be placed in the ten thousands place to make the number the greatest 5-digit number.

d) 2: This option is incorrect because the digit 2 cannot be placed in the ten thousands place to make the number the greatest 5-digit number.

**Correct Option:**

The correct option is a) 5 because the digit 5 can be placed in the ten thousands place to make the number the greatest 5-digit number. Therefore, the greatest 5-digit number is 99,999.