Read the sentence.

When I told the doctor of my sinus pain and nasal pressure, she offered me a decongestant.

Based on the prefix, de-, what does the word "decongestant" mean?

a device that causes congestion

a resource that defines congestion

a way to measure congestion

a substance that reduces congestion

Let's look at some other words with the prefix "de":

-decolonisation, the process through which countries were no longer colonies

-denazification: the process trhrough which people were taught to no longer be Nazis;

So the prefix "de" means that something will no longer be this thing: the answer is "a substance that reduces congestion"