Which of the following is a topping on the club avocado chicken?

The correct answer is guacamole, as it is a classic and essential topping specifically associated with the club avocado chicken dish. Guacamole contributes a creamy texture and rich flavor that complements the chicken and other ingredients, aligning perfectly with the theme of a dish centered around avocado.

In the context of the other options, while ranch dressing might be a common accompaniment for various dishes including salads or chicken, it is not specifically a topping for the club avocado chicken. Fried onions and barbecue sauce are also not typically used with this dish, as they do not align with the flavor profile intended for a meal that highlights avocado and chicken.