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.