Explanation
Step elements are used to organize the OmniScript into one or more pages. They define the layout, navigation, and visibility of the OmniScript elements. They can also have pre- and post-actions that can execute DataRaptors, Integration Procedures, or other processes before or after the user completes the step.
The other options are not correct because:
Allows the user to input data: This is not the purpose of Step elements, but of Input elements, such as Text, Number, Date, Picklist, etc.
Groups elements that extract data: This is not the purpose of Step elements, but of Data Source elements, such as DataRaptor Extract, Integration Procedure Extract, etc.
Enables the use of repeatable blocks: This is not the purpose of Step elements, but of Repeatable Block elements, which allow the user to add or remove multiple instances of a group of elements.

Explanation
The OmniStudio tool that the consultant should recommend to generate the pre-filled PDF is DataRaptor. A DataRaptor is a tool that can read, transform, and write data from Salesforce objects or JSON objects. A DataRaptor Load can write data to a PDF template using a PDF mapping file, and generate a PDF document with the contact information pre-filled. The PDF document can be stored as an attachment or a content document in Salesforce