If I wanted to replace "to slump" with a phrasal verb, which would you suggest me among these in this sentence "to flake off", "to flake out", "to drop in", "to stretch out"?
- I was so tired that I slumped on the sofa and went to sleep.
Could "flake off" work?
not a teacher
I'm not disagreeing with bhaisahab, it may just be a regional difference, but in my experience "flake out" has always meant "fall asleep".
On that basis I would tend to say:
I was so tired that I flaked out on the sofa.
I was so tired that I stretched out on the sofa and went to sleep.
"To flake out" means "to fall asleep [because you're very tired]" to me too.
I might say "I flopped on the sofa", "I fell facefirst onto the sofa".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
not a teacher
You could be right about regional differences. In my experience, to 'flake out", means to not follow through on a commitment. For example, "He flaked out on us and didn't even show up yesterday". A "flake" is a person who can not be trusted.
Here, an unreliable person is similarly described as "flakey" or a "flake", but "he flaked out on us" would be understood as "he didn't (do whatever) with us because he went to sleep".
a) Did John go to the party with you last night?
b) No, he was really tired and flaked out on us after dinner.