difference between "gaffe" and "blunder"

[Meral]

Synonym Search, Intermediate level

ESL/EFL Test #115 "Synonyms for mistake and blunder", question 8

Julia married to an old rich man when she was only eighteen because all she wanted was to be rich. Years later she realized it was a big ..........

(a) error
(b) blunder
(c) inaccuracy
(d) mistake

Synonym Search, Intermediate level

ESL/EFL Test #115 "Synonyms for mistake and blunder", answer 8

Julia married to an old rich man when she was only eighteen because all she wanted was to be rich. Years later she realized it was a big mistake.

Correct answer: (d) mistake

Your answer was: correct
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Hello,
Could you tell me the difference between "gaffe" and "blunder" please?

Meral

[Mister Micawber]

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A gaffe is a social blunder.
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Native English teacher at Mister Micawber's

[Babi]

I don't understand why I cannot use "blunder" here. Can you explain it to me?
Thank you in advance.

[Mister Micawber]

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Blunder = a gross, stupid, or careless mistake. I don't think that the mistake by the lady in the sentence can be characterized in any of those ways.
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Native English teacher at Mister Micawber's