The correct answer is Dialogflow API because it is designed for goal directed conversations that detect user intents and extract required entities in order to complete a task.

Dialogflow provides intents, entities, and slot filling so it can require all necessary parameters before fulfillment. In the bakery scenario it would recognize the order intent, capture cake size and flavor, realize that the delivery address is missing, and then prompt the user for that address. Once all required details are gathered it can hand off to fulfillment to place the order.

Vertex AI Search focuses on enterprise search and retrieval augmented experiences over content. It does not provide native intent modeling with required parameter collection to drive a structured ordering dialogue.

NaturaI Language API offers text analysis such as entity extraction, sentiment, and syntax. It does not manage multi turn conversations or enforce required slots to complete the task.

Gemini API enables general purpose generative and multimodal capabilities and can be orchestrated to build assistants, yet it does not include a built in framework for intents, entities, and slot filling. The question asks for a service designed for this type of dialog management which points to Dialogflow.

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