"What is Jack doing?"
"Jack" is a proper noun, which refers to a specific person named Jack. Without more context, it is impossible to determine what Jack is doing.
Here are a few examples in Engpsh:
Jack is playing soccer.
Jack is studying for his exams.
Jack is cooking dinner.
Translated into Chinese:
杰克正在踢足球。
杰克正在为考试做准备。
杰克正在做饭。