To configure a journey using Path Optimizer with a holdback of 60% of the contacts until a winner has been selected, the Marketing Cloud admin needs to set:
A). Data Extension entry source: Ensures the journey is pulling contacts from a specific data extension which can be effectively managed and monitored for the experiment.
D). Winner evaluation: This setting allows the admin to specify how and when the winning path is determined, which is critical to managing the holdback and subsequent path optimization.
Reference: Salesforce Help - Path Optimizer