We had taken great pain to check the structural soundness of the coachwork.

The grammatical error of the sentence is in option B. The meaning of a few nouns in plural form is different from their meaning in the singular form. ‘Pain’ is such a word. ‘Pains’ means ‘great care or trouble’ while ‘pain’ means highly unpleasant physical sensation caused by illness or injury. Here, someone had been involved in trouble or we can say, someone had taken a difficult responsibility to check something. Thus, replace “pain” with “pains” to make the sentence grammatically correct. Correct sentence: We had taken great pains to check the structural soundness of the coachwork.