请问It was not until the 1990s,when Mr.Wang was already in his

选择：D that
解析
一、句子总体分析
主句 It was not until the 1990s ___ he began to teach himself English
插入性时间状语从句：when Mr. Wang was already in his forties
二、所表达意思与句子结构分析
1、主句的意思是“直到上世纪九十年代他才开始自学英语”.
2、各种不同的表述手段——
* 正常语序：He didn’t begin to teach himself English until the 1990s.
* 倒装句：Not until the 1990s did he begin to teach himself English.
* 强调句：It was not until the 1990s that he began to teach himself English.
——其结构为： it（强调代词）+ be + 强调部分 + 助动词 + 主语 + 谓语动词 +其他部分
* 其他手段：It was in the 1990s when/before he began to teach himself English.
——其结构为：
主句：It was in the 1990s ——it 在这里是非人称代词,用来指代时间
从句：when/before he began to teach himself English.

结论：选择 A befor e和 B when 与原题结构有出入；C since 不能表示这种意思.