What type of operating system allows two or more programs or apps to reside in memory at the same time?

There are two types of operating systems:

1. Single tasking operating system which allows to run only one application at a time.

2. Multitasking which allows allows two or more programs or apps to reside in memory at the same time.

The type of operating system depends on its architecture. Different architectures allow to use different amount of RAM (Random Access Memory)