Is this training center ___ you said in your letter we were

这是一表语从句,即相当于you said in your letter we were to study English( in the training center) 可见从句中说的是地点.所以用副词where引导.
如果上面主句改为 Is this the training center-- 那后面就是定语从句可选的有where,和in which两个.这句的重点是主句为疑问句.要先把它变为肯定句才能看出后面是哪种从句.
this training center is (where) you said in your letter we were to study English.这和训练中心就是你在信中说的我们要学英语的地方吗?

地点状语从句，引导词用where=in which