教你一个简单实用的办法.
把你的句子还原到非强调句------ At the very beginning,Mr Fox made the decision THAT we should send more firefighters here.从非强调句,我们可以看出,at the very beginning是时间状语短语,不是句子,所以when就根本用不进去.
如果一定要选择B,句子必须是：
---------It was the begiining of this year WHEN Mr Fox made the decision THAT we should send more firefighters here.----------这是时间状语从句+同位语从句.when引导的是时间状语从句.（句子里面没有介词AT!）
It was AT the very beginning THAT Mr Fox made the decision THAT we should send more firefighters here.------多了介词AT,句子就成了强调句.
祝你学业天天向上.