Alt er love.
I saw "allting" used for "everything" in another exercise. Can i use it here too, i.e. "Allting er mulig."?
Yes, that should be correct, but wouldn't be as common as 'alt'.
why isnt mulig in the plural form?
"Everything" (and "alt") is a singular collective noun. For example, in English, you would say "Everything is fine". You wouldn't say "Everything are fine".