The correct option is
B Sure, in a moment
In the given conversation, the guest is requesting the receptionist to call a taxi for him.
Option A: This is incorrect as taxi is a non-living thing. It is not a person and one does not address a taxi as 'Hi Taxi'.
Option B: The sentence, 'No, I'm afraid not' is a possible response to a question which describes a person. Eg: The question, 'Are you fine?' can be answered by the response, 'No, I'm afraid not'. Hence, this option is incorrect.
Option C: This response means that the receptionist is assuring the guest that he will call a taxi shortly. This is an appropriate response to the guest's question and meaningfully completes the given conversation. Hence, option C is correct.
Option D: This is an incorrect response by the receptionist. This is because the receptionist is being requested by the guest to do something i.e 'call a taxi'. He cannot reply saying, 'Please do' to the guest. Hence, this option is incorrect.