Which of the following would be considered a Junction Object in Salesforce?

A junction object in Salesforce is specifically designed to facilitate a many-to-many relationship between two objects using master-detail relationships. When you create a junction object, it has two master-detail relationships to different objects. This design allows for the association of many records of one object to many records of another object, which effectively creates a many-to-many relationship.

In this context, the junction object acts as an intermediary entity that holds the connections between the two other objects. For example, if you have 'Students' and 'Courses', a junction object could be 'Enrollments', which links records from both the Students and Courses objects, allowing you to track which students are enrolled in which courses.

The other options do not fulfill the criteria for being a junction object. For instance, a custom object with a lookup to a standard object or two standard objects with a lookup relationship only enables a one-to-many relationship rather than a many-to-many setup. Thus, option C stands out as the correct answer because it accurately represents the structural requirement of a junction object in Salesforce.