或
I want to know what (is) the most (common) (means ) of transportation.
或
I want to know what (is) the most (common) (forms ) of transportation.
參考資料：
火車是壹種安全的交通工具。
The train is a safe means of transportation.
最快的交通工具是飛機。
The quickest means of travel is by plane
離開機場的交通工具通常是什麽?
What are some of the common means of transportation to leave the airport?
是啊，這是我最常用的交通工具。
Yes, this is the form of transportation I use most often.
Means of transport
牛津初階英漢雙解詞典（第二版）-...
Space外層空間
Means of transport交通工具
Electronics電子設備 ...