What does unwittingly mean?

[Witty]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #543 "English Slang Idioms (144)", question 8

The defendant's attorney had unwittingly given the District Attorney's office the case on a silver ......... by divulging a fact, in passing, that turned out to be the missing piece of the puzzle in regard to how the crime was committed.

(a) spoon
(b) letter
(c) dollar
(d) platter

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #543 "English Slang Idioms (144)", answer 8

The defendant's attorney had unwittingly given the District Attorney's office the case on a silver platter by divulging a fact, in passing, that turned out to be the missing piece of the puzzle in regard to how the crime was committed.

Correct answer: (d) platter
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unwittingly

[Mister Micawber]

unwittingly
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Native English teacher at Mister Micawber's