A选项的问题是比较对象不是一类，比较对象是now和at any time，现代科技能使小车”在现在比在过去”省油，语义存在逻辑错误；today和Now重复；at any time没有将自身排除在外，应为any other time
B选项的问题也是比较对象不是一类，比较的是small cars和they were。另外they是指上文提及的东西，放在这里变成谈及的small cars和这些cars在历史上的比较，显然语义也不正确
---------------------
I'm going to make air conditioners quieter. --> I'm going to modify existing air conditioners so that they make less noise.

I'm going to make quieter air conditioners.
I'm going to make air conditioners that are quieter.
--> I'm going to make a new model that makes less noise.

A is like the first one of these, so it doesn't make sense in context.

---------------------
首先排除DE，more fuel-efficient small cars有歧义，是“更多”的fuel-efficient small cars，还是“更节能”的small cars？
B they指代的是前面的small cars，是让同一批车子比之前更节能。排除
A 也是同一批small cars，“让车子现在比它们之前更节能”。排除
C that are more fuel-efficient than those at any other time in their production history是定语，修饰前面的small cars，意思是“能造出这样的小车，这些小车要比之前任何时候生产的小车更节能。”