Well first you need to show the rest of the problems before i can truely help you sorry.
The correct answer for the given question above would be the last option. The proper way to write a single digit whole number if it is in a sentence should be SPELLED OUT and not written in numerals or roman numerals. For example, I have SIX apples; and not I have 6 apples.
Simple
scientific notation
8.009 × 10-3