我觉得填 by which time 和 in which 都可以，只是意思有所不同。

要弄清这个问题，我们需要弄清以下几个相关问题：

▲by which time 与in which 的用法

如果先行词是指时间的话，by which time 与in which 的用法区别如下：

(1) by which time 意思是“（到）那时”“（到）这时”，通常是指到某个时间点，即它的先行词通常应是时间点。如：

We arrived at noon, by which time the demonstration was over. 我们是中午到的，这时游行业已结束。

He came back eight months later, by which time the Japanese had decided to say yes. 八个月后他回来了，而当时日本人已经决定同意。

(2) in which 意思是“在此期间”，即它的先行词应该是时间段。如：

Galileo's ideas were well in advance of the age in which he lived. 伽利略的思想远远超越了他所生活的时代。

You have 14 days in which you can cancel the contract and get your money back. 你可在14天之内取消合同，收回钱款。

▲which 先行词如何确定

The next Olympic Games are in two years, ________ I will be a university student.

在你的句子中，which 指代的先行词是哪个呢？是 two years 不是 in two years？

(1) 如果是先行词 two years（两年），那它是时间段，那其后就应该用 in which，即：

The next Olympic Games are in two years, in which I will be a university student.

下一届奥运会要等两年后，在这两年当中我将在要上大学了。

(2) 如果是先行词 in two years（两年后），那它是时间点，那其后就应该用 by which time，即：

The next Olympic Games are in two years, by which time I will be a university student. 下一届奥运会要等两年后，到那时（指两年后）我将在上大学了。

▲当先行词为“介词+一段时间”时，后面的定语从句可以用 in which吗

我在网上查了一下，你的这道题在网上流传很广，而且答案都是清一色的选 by which time，而把 in which 排除在外，显然是一种误传。比如下面一句就是摘自权威词典，其中的 after three days 与你这道题中的 in two years 用法和意思相近，但它的后面接的就是 in which：

Impatience is growing after three days in which nothing has been achieved. 三天没有任何进展，烦躁情绪见长。（摘自柯林斯高级英语词典）

▲个人揣测

网友的这道题的命题过程可能是这样的：命题者根据 The next Olympic Games are in two years, by which time I will be a university student. 这个句子命制了上面这道题，命题者根据原句给出的参考答案是 by which time，另外三个选项是命题者自己设计的，但由于考虑欠周全，结果不小心设计了一个“正确”答案作为干扰项，所以导致了这样的结果。

个人看法，仅供参考！