To answer this question, the user needs to have some basic knowledge of Informatica.

In Informatica, a reusable component is a type of component that can be used in multiple locations within the same project or across multiple projects.

Now, let's look at each option and determine which one is a reusable component:

A. Mapping: A mapping is a set of instructions that defines how data is transformed from source to target. While a mapping can be reused in the same project, it cannot be reused across multiple projects. Therefore, mapping is not a reusable component.

B. Mapplet: A mapplet is a reusable set of transformations that can be used in multiple mappings. Mapplets are designed for reuse and can be used across projects, making them a reusable component.

C. Session: A session is a set of instructions that defines how a mapping is run, including the connections, sources, targets, and transformations. While a session can be used in multiple mappings within the same project, it cannot be reused across multiple projects. Therefore, session is not a reusable component.

D. Workflow: A workflow is a set of instructions that defines how and when a session or a set of sessions is run. While a workflow can be used in multiple mappings within the same project, it cannot be reused across multiple projects. Therefore, workflow is not a reusable component.

Based on the above explanations, the correct answer is:

The Answer is: B. Mapplet