For the scenario described, where a loyalty program wants to send an immediate notification after a voucher is added to a member's account, the entry event in Salesforce Marketing Cloud needs to trigger off of the creation of a record in Salesforce that contains the necessary data attributes for personalization.

Option C is the most suitable choice because it specifies an entry event based on the "Voucher," "Contact," and "LoyaltyProgramMember" objects. This approach aligns with the requirements because it directly utilizes the "first name" from the Contact object and the "membership number" from the LoyaltyProgramMember object, which are necessary for personalizing the notification message.

Moreover, Option C would likely require the least amountof development effort because it leverages existing Salesforce data and related objects without the need for additional configuration or data extensions. In contrast, other options would require the setup of a data extension or custom objects that may notbe directly tied to the triggering event, thereby increasing the complexity and development effort.

In Salesforce Marketing Cloud, Journey Builder is the tool that would be used to create this customer journey. It allows for the configuration of entry events based on data changes in Salesforce, such as the addition of a voucher to a member's account. The real-time nature of this trigger is essential for the immediate notification requirement.

According to Salesforce's official documentation, using Salesforce data as the entry source for a journey allows for real-time messaging based on record creation or updates. This means that as soon as a voucher record is created or updated in Salesforce, the entry event can trigger and begin the journey, which will sendthe personalized notification to the member.

In practice, setting up this entry event in Journey Builder would involve specifying the criteria for the trigger (in this case, the creation of a voucher) and mapping the necessary data fields for personalization. This is consistent with Salesforce's best practices for creating personalized, real-time customer engagements through Journey Builder.

Salesforce's documentation on Journey Builder and real-time event triggers provides further details on how to configure these types of journeys and can be referred to for step-by-step guidance and the latest feature updates.