Modifying the software by restructuring is called

Explanation

This question belongs to the familiar four-way set in which each maintenance type is identified by its trigger. Perfective maintenance covers changes made to improve the software itself – restructuring the code, improving performance, readability or maintainability, and adding enhancements the users ask for – without any fault having been reported. Corrective maintenance fixes reported defects, adaptive maintenance keeps the software working as its hardware, operating system or external environment changes, and preventive maintenance makes changes to head off problems that have not yet appeared. The official key marks perfective, option 3.