Final answer:
The most likely answer is a beef patty with barbecue sauce, considering Texas' preference for beef in BBQ and the common use of sweet, dark BBQ sauces like those from Kansas City.
Step-by-step explanation:
When considering the 2nd protein on the BBQ burger mentioned, along with the sauce and vegetable, it is important to consider regional BBQ practices and preferences. In Texas, beef is the most popular BBQ meat due to the geography and cattle ranching practices, making the beef patty a common choice for a BBQ burger. The sauces can vary by region: Kansas City is known for its sweet, dark BBQ sauce; South Carolina has a mustard-based sauce; East Carolina uses a vinegar-and-hot pepper sauce; Louisiana has cayenne pepper flavored hot sauce; and Memphis prefers a dry rub. Considering these factors, the most likely answer would be a beef patty with barbecue sauce because of the popularity of beef and sweet BBQ sauce in burgers.