Final answer:
In AP style, the correct way to write the decade of the 1990s is 'the 1990s' or 'the nineties'.
Step-by-step explanation:
In AP style, the correct way to write the decade of the 1990s is 'the 1990s' or 'the nineties'. It is important to note that in AP style, numbers from zero to nine are spelled out, while numbers 10 and above are written as numerals. Therefore, 'nineties' is spelled out as it refers to a decade.