Identifying incorrect part of sentence: Thousands of lives will be in stake if emergency aid does not arrive in the city soon.

• Complete Sentence: Thousands of lives will be at stake if emergency aid does not arrive in the city soon.
→ Translation: শিগগিরই জরুরি ত্রাণ পৌঁছাতে না পারলে হাজার হাজার মানুষের জীবন বিপন্ন হয়ে পড়বে।

• at stake (idiom)
English Meaning: If something that is valuable is at stake, it is in a situation where it might be lost.
Bangla Meaning: ঝুঁকির মধ্যে।

Example Sentence:
- Thousands of lives will be at stake if emergency aid does not arrive in the city soon.
- The success of the project is at stake, so we need to ensure that every detail is carefully considered.
- The environmentalists emphasized that the future of the planet is at stake if urgent action is not taken to address climate change.

Source:
1. Cambridge Dictionary.
2. Accessible Dictionary by Bangla Academy.