.in which 是正确选项.
题目been后面打少了个made,完整的句子是：The science of medicine,【in which 】progress has been 【made】 very lately ,is perhaps the most inportant of all sciences.这是个非限制性定语从句,先行词The science of medicine,关系代词in which,in which逻辑上是in the science of medicine.
把原复合句分拆为两个简单句是：The science of medicine is perhaps the most important of all the sciences.Progress has been made in the sciece of medicine very lately.