Answer: Drusilla will replace both again after 18 months.
Explanation: If Drusilla replaced both the light bulb and the air filter in September 2019, she will replace both again in March 2021, which means she will replace them together after 18 months (a year and a half). What explains this result is the fact that the least common multiple of 9 and 6, that is to say the smallest multiple of these two numbers, is 18. Therefore, the correct answer is 18 months.