答案：D

该句是复合句，主干部分为there was a very interesting remark in a book，介词短语by an Englishman和定语从句that I read recently都修饰a book，分词结构giving...在句中作伴随状语，what he thought was a reason for...作giving的宾语，what he thought又作这个宾语从句的主语。