It was ---- a ruler ---- he spent two yuan.

你分析的不完全对. 选B
这里确实是考部分强调句, 但你没看出来的是还有一个spend 的用法: spend ... doing sth.
所以这句话的正常语序是: He spent two yuan buying a ruler.
我们对buying a ruler 部分时行强调, 就变成了: It was buying a ruler that he spent two yuan.