In statement A, "they" in second part is incorrect. It should be in accordance to the singular object present in first part of the sentence.

In statement B, it should be "The people in my country are smarter than those in other countries.".

In statement C, it should be "The number of failures of product in the final quality inspection are increasing every year.".

Only statement D doesn't have any error. Hence, option D is the correct answer.