meaning of "out like a light"

[Mendy]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #546 "English Slang Idioms (149)", question 7

His wife purchased a new outdoor furniture set. It was the kind that required assembly at home and included detailed instructions on how the assembly was to be completed. She anticipated that her husband would put the set together once she got home. However, much to her chagrin, her husband was out like a ......... and snoring loudly when she got home.

(a) fuse
(b) candle in the wind
(c) burnt bulb
(d) light

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #546 "English Slang Idioms (149)", answer 7

His wife purchased a new outdoor furniture set. It was the kind that required assembly at home and included detailed instructions on how the assembly was to be completed. She anticipated that her husband would put the set together once she got home. However, much to her chagrin, her husband was out like a light and snoring loudly when she got home.

Correct answer: (d) light
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out like a light means what?do explain with some more examples.

[Mister Micawber]

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There are several pages with definitions and examples HERE.
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Native English teacher at Mister Micawber's