OG详解-OG7 文法 Q2

Choice D is the best answer. The convention being tested is the use of determiners in a sentence. The plural determiner “these” agrees in number with the plural noun “letters” that it modifies. This choice clearly indicates that the letters demonstrate Alcott’s business sense. 
Choice A is incorrect because the singular determiner “one” doesn’t agree in number with the plural noun “letters.” Choice B is incorrect because the singular determiner “that” doesn’t agree in number with the plural noun “letters.” Choice C is incorrect because the singular determiner “this” doesn’t agree in number with the plural noun “letters.”