When White opens with e4, what do you play as Black?
I will play 1...e5. Because, this opening uses second only to Sicillian Defense by master game standards and continues to be used from beginners to master games. It's not too complicated, and there's not much to memorize.
Interesting take... what makes you think this?
I'd experiment with them all and stick with the one you like more. As a personal guideline, I would try e4 e5. It's what I used for years and how I first crossed 2000 rapid. I also dabbled into the French Defense on and off before I was 1500 rating, but I gave up on it for a few reasons, but mostly me not liking the cramped space. I would probably avoid the French Defense at the lower levels because the piece development isn't as smooth and instructional.
Scandinavian is also a bit dubious even if it's playable.
I'd choose e4 e5, but Sicilian, or Caro-Kann are fine options as well. Try what you want and stick with it.