---- he spent much time on his spoken English(act)

Actively 积极的
如不看要填的空,则这句话就是 he spent much time on his spoken English.这句话是完整的.因此要加上一个词,只能是修饰句子或其中一个成分的副词.
这样想的话,act的副词只能是actively.
连起来再看,解为“积极的”,还比较通顺.
个人浅见.