* No request items are committed to the target viewpoint, and the subscription assignee is notified that an issue must be resolved before any items can be committed: This option is correct because when a subscription is triggered by changes in a source viewpoint and auto-submit is enabled, the request isautomatically submitted for approval. However, if one or more items in the request are invalid, the request is not approved and no items are committed to the target viewpoint. The subscription assignee is notified that an issue must be resolved before the request can be approved and committed.

Explanation
When you are mapping nodes from dimensions in two source applications to a dimension in a single target application, you need to set up separate hierarchy sets for each source-to-target relationship. This way, you can define the mapping rules and node type converters for each source node type and target node type pair. You cannot use target nodes and converted source nodes in two separate hierarchies, because this would not establish a mapping relationship between them. You cannot use target nodes in one hierarchy set and converted source nodes in a separate hierarchy set, because this would not allow you to export the mappings to the target application. You cannot use target nodes as parents and converted source nodes as children, because this would create a hierarchical relationship instead of a mapping relationship. References: Working with Hierarchy Sets - Oracle Help Center ; Creating Mapping Viewpoints - Oracle Help Center2

Explanation
When you want to map two general ledger systems, GL1 and GL2, to a single EPM Planning application, and it is time to synchronize changes by exporting the plan account mappings, the following statement about mapping keys is true: you need two mapping keys, one for GL1-to-Planning and one for GL2-to-Planning.
When you export a mapping, you select the location with its associated mapping key. Mapping keys are used to specify the source node types mapped to target node types and to define a location name to export the mapping data. You need to define one mapping key for each source node type mapped to a target node type.
For example, if you map two source applications to one target application, you need to define two mappingkeys defining the mapping relationship from each source node type to the target node type. Each mapping key is identified by a unique location name that you enter. The location name is used to export the mapping data and by the consuming or external application to import the mapping data. You do not need only one mapping key and location to export the plan account mappings, because this would not specify the correct source-to-target relationships. You do not need to create shadow mapping keys in the target application or one location per mapping key, because these are not supported by Oracle Enterprise Data Management Cloud.
References: Defining Mapping Keys - Oracle Help Center1; Exporting Mapping Data - Oracle Help Center2