What is an informal high-level description of a program's operating principle known as?

The correct choice is Pseudocode, which is an informal high-level description of a program's operating principle. Pseudocode allows developers to outline the logic of a program using simple language that resembles programming syntax but does not adhere to the strict rules of any specific programming language. This makes it easier to focus on the flow and structure of the algorithm without getting bogged down in syntax and technical details.

Pseudocode serves as a valuable tool during the planning phase of software development, as it helps in visualizing how different parts of the program will interact and operate. This abstraction helps developers communicate ideas clearly, brainstorm, and lay out the steps in the program logic in a way that can be easily translated into actual code later on.

In contrast, source code refers to the written program in a specific programming language that can be compiled or interpreted. An algorithm is a well-defined set of rules or steps to solve a problem, which may be described in pseudocode or implemented in actual code. A flowchart, on the other hand, visually represents the flow of control through a system or process using geometric shapes and arrows.