Correct option is B
Option (b) is the correct meaning of the given idiom.
Idiom Meaning: “The last straw” (or "the last straw that broke the camel's back") means the last in a series of problems or annoyances that finally causes someone to lose patience. (किसी परेशानी की श्रृंखला में अंतिम सहनशीलता की सीमा)
Example: When he arrived two hours late again, it was the last straw—I fired him.
Other related idioms and their meanings:
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The tipping point – The moment of critical change.
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Push over the edge – To drive someone into breakdown or anger.
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At the end of your rope – Being out of patience or options.