We told him not to lose heart ____difficulty.

A in times of   B at the time of  C before  D when

此题最佳答案是A。B和C都不合适。

此题有一定的难度，请看以下解答。

一、in times of + 抽象名词

该短语接的抽象名词，一般为undesirable 类的名词，如“困难，麻烦，战争，紧急情况”等。往往表示“某个不良的时期或阶段”。

1. They always helped us on in times of difficulty.

在我们遇到困难时,他们总是鼓励我们。

来自《简明英汉词典》

2. Exceptionally, in times of emergency, we may send a team of experts.

在紧急情况下，我们可能会破例派一个专家小组。

来自柯林斯例句

3. In times of war, governments often sweep human rights aside.

战争时期，政府经常对人权置之不顾。

来自柯林斯例句

4. Members of a family should cling together in times of trouble.

一家人应该患难与共。

来自《简明英汉词典》

5. In times of financial stringency it is clear that public expenditure has to be closely scrutinized.

财政紧缩时期，无疑要对公共支出进行严格审查。

来自柯林斯例句

6. In times of currency crisis interest rates can be raised as a sign that a government is in command.

在发生货币危机的时期，利率会提高，以表明政府有控制力。

来自柯林斯例句

7. It also stockpiled food to give relief in times of famine.

政府还准备储备物资以防灾荒。

来自英汉非文学 - 文明史

8. Many fail, especially in times of recession, but others take their place.

许多公司倒闭了, 特别是在经济衰退时期, 但也有不少公司站稳了脚跟。

二、at the time of + 事件名词/动名词/人名

该短语接的这类名词，往往有如下特点：

1. 表示具体事件的名词，如：事故，爆炸，凶杀，死亡等；

2. 表示具体动作的动名词；

3. 某个人物（的时代）

1. The house was unoccupied at the time of the explosion.

爆炸发生的时候房子里没有人。

来自柯林斯例句

2. He had fled from France at the time of the persecution.

他在大迫害时期逃离了法国。

来自《简明英汉词典》

3. He was sane at the time of the murder.

在凶杀案发生时他的神志是清醒的。

来自《简明英汉词典》

4. What were you doing at the time of the earthquake? 

地震时你在干什么?

5. It happened at the time of King Alfred. 

它发生在阿尔弗雷德王的时代。

6. She was away at the time of the accident. 

发生意外时她不在。

7. We check prices at the time of going to press.

我们会在付印的时候核对价格。

来自柯林斯例句

8. It is very important that every sale of a dangerous poison should be entered up at the time of sale.

出售的有毒物质应该登记出售时间,这是很重要的。

来自《简明英汉词典》

▲“在困难面前”，一般指具体的困难。应说：before a difficulty 或 before difficulties，不说before difficulty。

综上，根据你给的句子，difficulty为抽象名词，不是具体事件名词，适合跟in times of 搭配。