Final answer:
Existing Xfinity customers might be eligible for a referral bonus when they add services such as mobile phone service or home security, depending on the company's current promotions.
Step-by-step explanation:
The question refers to the promotion or bonus that Xfinity customers can receive when they add a particular service. The services listed are: A) Cable TV, B) Home security, C) Mobile phone service, and D) Streaming service. To answer this question accurately, one would need to refer to the current promotions offered by Xfinity. However, it's common for telecommunication companies like Xfinity to offer referral bonuses for services that are growing and expanding, such as mobile phone service or home security. Streaming services typically have their own subscription models, while cable TV is a more traditional service that many customers already have.
Therefore, without the exact current details from Xfinity, it's challenging to provide a definitive answer. For educational purposes, if a referral program does exist, it's likely tied to a service where Xfinity is trying to increase their market share or customer base, such as mobile phone service or home security. To obtain the correct answer, one should check Xfinity's latest offers or contact their customer service.