Our cat has gotten really bad fleas, due to it being super humid here as well as the rain. She is strictly an indoor cat, so she is exposed to anything else.
Advantage II is the right route for you if you only have fleas and if your cat has ticks as well then the Frontline Plus would be the other you would need to get. Advantage II is a lot cheaper than the Plus.
Check out https://www.chewy.com/ My daughters fur babies take this one.
I prefer Advantage II. Frontline stopped working on my cats years ago
Wal-Mart has a knock of avantage, it works just as good but I can't remember the name but Advantage isbetter
Advantage is what my neighbor uses and it works well.
We use front line plus but it really depends on the fleas in your area.