Which of the following objects can be used in an expression rule?

In the context of Appian expression rules, literal values are indeed objects that can be used. They are fixed values defined directly in the expression, such as numbers, strings, or booleans. For instance, a number might be represented as 5, a string as "Hello", or a boolean as true. When creating expressions within rules, these literal values serve as constants that help to define conditions or return specific data types as results.

Conditions, variables, and data types play different roles within the context of expression rules. Conditions refer to logical statements that evaluate to true or false, and while they are essential for controlling the flow of logic, they aren't objects themselves that can be directly utilized as components of expression rules. Variables temporarily store values during the execution of an expression but are not predefined objects like literal values. Data types, on the other hand, classify the nature of the data being handled (such as integer, text, or list), but they do not represent usable objects within an expression in the way literal values do. Thus, the ability to use literal values directly aligns with the framework of expression rules in Appian, making this option the correct choice.