—There were already five people in the car but they managed to take me as well.

“车上本来已经有五个人了，但他们还是设法把我也带上了。”

这句话是站在现在的角度描写过去的事的，说话的人跟听者描述过去发生的事。

那么，你的回答，当然是你站在现在的角度对过去的情况进行猜测：

—It couldn't have beena comfortable journey.

“那不会是一个舒服的旅程。”

所以，这个是对过去的事进行猜测。而 can/could have done 的作用之一就是表示对过去的事情进行的猜测是，所以选D。    

赞同justik网友的解答。站在现在的立场对过去的情况进行推测，必须使用情态动词+不定式的完成体have done的形式。情态动词本身用现在或过去形式与时间无关，只是推测时说话人的信心程度不同。must不能用于否定的推测。因此只能选择D。

— There were already five people in the car but they managed to take me as well. — It couldn't have been a comfortable journey. A. can't be B. shouldn't be C. mustn't have been D. couldn't have been 

couldn't have done 对过去发生的事情作否定推测，意思不可能做某事

must have done 对过去发生的事情作肯定推测，意思一定做某事