What does this idiom mean: Opportunity knocks but once?

[mansour abdellah]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #546 "English Slang Idioms (149)", question 5

When his wife asked him why he was willing to sell their house at a loss and move to another country just for a job, he told her, "This job is our ticket to making serious money and early retirement. I have to take it. ......... knocks but once."

(a) Chance
(b) Fate
(c) Winning the lottery
(d) Oppurtunity

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #546 "English Slang Idioms (149)", answer 5

When his wife asked him why he was willing to sell their house at a loss and move to another country just for a job, he told her, "This job is our ticket to making serious money and early retirement. I have to take it. Oppurtunity knocks but once."

Correct answer: (d) Oppurtunity
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what is the correct answer ? give me explanation

mansour abdellah

[MrPedantic]

Hello MA,

"Opportunity knocks but once" is a well-known proverb.

("Chance knocks but once" is also sometimes heard, but it is much less well known; while "Fate" tends to knock more than once, as at the beginning of Beethoven's Symphony No. 5.)

Best wishes,

MrP

[Yankee]

.
Note also MrP's correction of the spelling: opportunity.

(The test contains a typo.)
.

[Linda]

That typo has been fixed.
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