As soon as I could write, I ____ them down on paper.

put ..down写下记下 语意中就是在写东西
take down 一般为取下的意思,没有put 可以选took

我觉得选put down，貌似take down后面一般不会加on paper这样的介词短语。

选B。
put down=write down写下
take down the speech记下这个演说

选took, 有“记下”的意思