1.答案：A.
2.翻译：说到成功,我并不是想指当这个字眼(即成功)被提到时,而通常被(人们所)想到的含义.
3.
1）固定短语：mean A by B 说到B时所指的含义
如：What do you mean by that?你说到那(个东西),
该题把by B(success)提前到句首了.
2）短语中的A即题中的what is usually thought of when that word is used.这是一个含有when引导的时间状语从句的名词性从句（确切地说,是mean的宾语从句）.
至于该宾语从句的意思,是指“当那个字眼被用及时,通常被想到的（意思/含义）”.
3）该宾语从句中的what is,做该从句的主语和系动词.这样从句的结构才能完整：what is ususally thought of (when that word is used).