You should try to practice speaking English as much as possi

as much as 比as possible as you can 侧重于量,有尽可能多的意思
as possible as you can 更宏观,包括尽任何努力
本句前面提到了try to practice speaking English ,练好口语是靠尽可能多说英语
如果用as possible as you can却是尽力学好口语（较含糊）,并不能准确表达说话人的原意“尽可能多的说”
从选择题的目的,“选最好的,不是合适的”来看,as much as possible 是更好的选择.