What is the smallest possible number that can be formed from the digits 9, 5, 4 and 8 if each digit is to be used exactly once?

1. 4589

The correct option is A 4589
To find the smallest possible number formed from the digits:

1) We place the smallest of these digits (4) in the thousands place.

2) We place the smallest of 9, 5 and 8 in the hundreds place.

3) We place the smallest of 9 and 8 in the tens place.

4) We place 9 in the ones place.

$\therefore$ The number is 4589.