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"

Best thing to do, reread whatever you are writing on, and highlight things that are scientific or anecdotal. then pick which ever is easiest for you