Answer:
starting with lowercase letters, using uppercase letters for the first letter in a new word
Explanation:
starting with a dollar sign, using lowercase for the remaining parts of each word
The underscore.
But actually you can use spaces in file names, in many file managers. I think it just shows you a space while it actually saves an underscore, or something like this.
Answer:
Ok, so it's not letting me submit a text response, so I'll attach an image of my answer.
Explanation: