Which type of variable is not recommended to be used in programs because they make programs hard to understand and debug?

Global variables are generally not recommended for use in programs because they can lead to code that is difficult to understand and debug. Since their values can be changed from anywhere in the program, tracking down the source of errors or unintended side effects becomes challenging. This lack of encapsulation can also hinder modularity and reuse of code. It's usually better to use local variables or pass variables explicitly to functions to maintain clarity.