People in different countries are different in what they eat and ___ they cook their food.

首先what they eat and the way they cook their food 这部分作为different in中in 的介词宾语.what they eat 和 the way they cook their food 相互间是并列句,and是用来连接这两个并列句的.what they eat 可以忽略不看,他是完整的并列从句中的一句完整句.
假设用你选定的B,填入句中,...different in the way in that they cook their food.in that 能充当从句中的什么角色呢?从句动词cook的宾语已经有了,是their food,in that的that也是宾语,和their food重复了,冲突了,所以不能用B.