What mobile operating system below requires all applications to be reviewed and approved before they can be made available in the App store?

The correct answer to the following answer will be iOS.

Explanation:

The iOS formerly stands for iPhone Operating System, basically is a mobile operating system developed by Apple Inc. This edition mainly supports Apple devices such as iPhone, iPod touch, iPad, etc.

Just because of this iOS gives the improvement to the entire application and is one of the operating systems which requires all that applications to be approved before showing availability in the App stores.

So, iOS is the right answer.