it was a hot evening ___the house caught fire A that B when

B when 是对的.
去掉it was.和空格以后,句子不通,说明不是强调句,排除A,that； Csince D before明显不对.
其实这是个时间状语从句,When the house caught fire,if was a hot evening.“房子起火是在一个酷热的晚上.”

首先排除CD,在此定语从句中，that evening作主语，所以不需要that,用when

B,WHEN作引导词，引导时间定语从句，千万不要看到IT IS以为是强调句，而错选A。