The correct answers are: I accidentally burned myself on the hot oven rack. The head chef himself told me my soufflé was a masterpiece! I managed to clean the entire kitchen disaster zone by myself.

I accidentally burned myself... is correct because the subject ("I") and the object receiving the action are the same person.

The head chef himself... is correct. Here, "himself" is used as an intensive pronoun to add emphasis.

...by myself. is correct. The phrase "by" + reflexive pronoun means "alone" or "without help."

Incorrect options:

"Please send your recipe feedback to the kitchen manager or myself" is incorrect. You should use the object pronoun me here. (Tip: Take out "the kitchen manager" and see how "send feedback to myself" sounds!)

"My sous-chef and myself will be preparing..." is incorrect. You need the subject pronoun I as part of the compound subject ("My sous-chef and I").