After defining a formula, the result must be stored and made available in the Data Mapper output structure. In the current Data Mapper design pattern, the formula result path creates a JSON node, and the developer then maps that result into the final output through the mapping configuration. This is why option B is the best answer: the formula is not useful to downstream actions until it is mapped to the required target output. The Extract tab is used to define source data retrieval, not to map formula results. The Output tab applies to specific Extract-style mappings in older terminology, but the question uses the generic Data Mapper mapping workflow. The essential step is to map the formula result into the JSON.

The Id Field for Actions in the FlexCard specifies which field from the FlexCard data source will be used as the record ID for the OmniScript. If this field is blank, the OmniScript will not receive any record ID and will not be able to retrieve any Salesforce data. In the DataRaptor Extract Action, the Input Parameters Filter Value determines how to filter the data based on the record ID. If this value is misspelled, the DataRaptor Extract will not be able to match the record ID and will not return any data.

The Node field for the parent FlexCard should be set to {Records}, which is an array of all the records retrieved by the parent FlexCard data source. This will allow the child FlexCard to access the records and display them in a DataTable.

A Set Values Action in a Value field and a Remote Action in an Additional Output value field can execute a function like CONCAT or DATEDIFF by using the syntax {{function(parameters)}}. For example,
{{CONCAT('Hello', 'World')}} or {{DATEDIFF('2021-01-01', '2021-01-31')}}.

Navigation button labels such as Next are controlled through OmniScript setup-level label mapping/custom label configuration, not by editing each Step element or creating Navigate Actions. The Next button is part of the OmniScript framework navigation, so changing it globally for the script belongs in the setup configuration where element names and labels can be mapped. A Navigate Action is used to navigate to another page or component; it is not the framework Next button. LWC Component Override is for replacing component rendering behavior with a custom LWC, which is excessive and incorrect for a simple label change.
Salesforce documentation describes OmniScript setup configuration and custom label references for OmniScript properties, supporting the setup-label approach.