我觉得答案应选 C。关键是这里的介词 in 不能省略，表示“在……季节里”。

in which to have outings 就是所谓的紧缩定语从句，相当于in which people had outings。

关于“紧缩定语从句”的用法，可参考英语语法网的以下文章：

http://www.yygrammar.com/Article/200906/1248.html 

It was a bad season ______.

A. to have outings

B. which to have outings

C. in which to have outings（带有逻辑状语的不定式作后置定语）

D. for us to have outings

选哪个？问什么？

答案解析：C为正确答案

其原因如下：

(1)It was a bad season in which to have outings.(带有1个逻辑时间状语的不定式作后置定语修饰前面的名词season)

(2)=It was a bad season in which we could have outings.(带有1个定语从句作后置定语修饰前面的名词season)

(3)=It was a bad seasson to have outings in .

(带有1个不定式作后置定语修饰前面的名词season)

参考译文：这是一个不适于郊游的季节。

(备注：以上介词in+关系代词which用来作不定式时间状语）

以上句子结构实际上市属于定语从句简化后的形式，简化后的介词“in”必须加上，不可以省略，例如下面：

这个穷人没有房子住。

The poor man has no house in which he can live.

=The poor man has no house to live in.

=The poor man has no house in which to live.

(备注：以上介词in+关系代词which用来作不定式地点状语）

这个乞丐没有钱购买食物。

The beggar has no money that he can buy food with.

=The beggar has no money to buy  food with.

=The beggar has no money with which to buy.

(备注：以上介词in+关系代词which用来作不定式方式状语）