3.The question is ________ can be put into practice.
(A)how you have learned
(B)how that you have learned
(C)that why you have learned
(D)how what you have learned

how that ～ 這裡的 that 引導完整的名詞子句 ( S + V + O )，

學了什麼並沒有指明，故不可以用 that。

而 how what ～ 這裡的 what 所接的為疑問句的變形，其子句本身只要符合 S + V 即可。

what have you learn ? → what you have learned ～

本句：

The question is how (what you have learned) can be put into practice. 

問題是你所學到的如何能夠被付諸實踐?

較少看到此用法，可拆解成：

1.what you have learned:(你所學到的)可視為主詞it。

2.The question is how it can be put into practice.問題是它(it)如何能夠被付諸實踐? 

3.所以一目瞭然，兩者合併就是最上面的提問。

A)how you have learned (試著代入，其不太符合語法,例如應：how the knowledge you have learned,要有個先行詞knowledge較妥)

(B)how that you have learned (試著代入，但其不太符合語法,例如應：how the knowledge that  you have learned,要有個先行詞knowledge較妥)

(C)that why you have learned (語法看起來沒錯,不過翻成中文不合意)
(D)how what you have learned (正確) 

interesting, this one!