From the following, which sentence is incorrect-

The correct sentence is - The books which/that were written by him are treasures for his countrymen.
(Those হচ্ছে Demonstrative Pronoun ‘that’ এর বহুবচন, কিন্তু এখানে বাক্যের চাহিদা অনুযায়ী Relative Pronoun- which/that বসবে।
মনে রাখতে হবে Relative Pronoun হিসেবে that এর বহুবচনও that.