Final answer:
Custom Apex Sharing Reasons in Salesforce can be defined for both custom objects and some standard objects known as top-level standard objects, and this statement is true.
Step-by-step explanation:
The statement that Custom Apex Sharing Reasons are available for Custom Objects and Top-Level Standard Objects is true. In Salesforce, Apex Sharing provides a programmatic way to share records according to complex, dynamic criteria that cannot be represented with the declarative sharing model. With Custom Apex Sharing Reasons, developers have the ability to define custom reasons for sharing records. They can do this only for custom objects and some standard objects called top-level standard objects, such as Account, Contact, Opportunity, Case, and others that directly support sharing.