Final answer:
Horatio receives an unexpected letter from Hamlet detailing his escape and return to Denmark, which does not match any of the provided options (A, B, C, or D).
Step-by-step explanation:
The kind of mail that Horatio received unexpectedly from Hamlet in Shakespeare's play is not explicitly given in the text provided for reference. However, based on the play 'Hamlet' by William Shakespeare, Horatio receives letters from Hamlet during the course of the play that contain surprising news about Hamlet's escape from a ship bound for England and his return to Denmark, which would best correspond to a letter of unexpected and surprising content rather than any of the specific options provided (A love letter, a declaration of war, a plea for forgiveness, or a letter of condolence). Therefore, the answer to what kind of mail Horatio received unexpectedly from Hamlet does not align with any of the options provided (A, B, C, or D).