What additional ingredient is included in the Sirloin kebab besides steak and vegetables?

The correct answer is mushroom cap. In the context of the Sirloin kebab, it is important to note that this dish typically features marinated steak pieces interspersed with various vegetables. The inclusion of mushroom caps adds a complementary flavor and texture, enhancing the overall taste and experience of the kebab. Mushrooms are a common ingredient in kebabs, as they can absorb the marinade and provide a juicy contrast to the grilled steak and vegetables.

The other choices do not align with the standard ingredients found in a Sirloin kebab. Rice, while often served as a side dish in many cuisines, is not a typical component of kebabs themselves. Jack cheese could potentially be used in other dishes at Texas Roadhouse, but it does not feature in the kebab. Sour cream, also mainly a condiment, does not serve as an ingredient in the kebab mix. Thus, mushroom caps stand out as the correct addition to the Sirloin kebab.