They have made much progress__(用）their hard work.
这个“用”,是“以.的方法”,with
如：With the help of the computer we can do lots of things.
用了电脑,我们可以做许多事（在电脑的帮助下）
by也可以是“以...”、“通过...”,by doing更常用,with sth.更常用
本句中their hard work是个名词,所以用with比by更好
但如果是asking often and learning,就用by更好