Final answer:
If the circulation around an airfoil is doubled with all other factors unchanged, the lift on the airfoil doubles according to the proportional relationship defined by the Kutta-Joukowski theorem.
Step-by-step explanation:
The question relates to the concept of lift generated by an airfoil, which is an important topic in the study of aerodynamics within Physics. According to the Kutta-Joukowski theorem, the lift L is directly proportional to the density ρ, the velocity V, the circulation Γ, and the span b of the airfoil. The relationship can be expressed by the formula L = ρVΓb. If the circulation around an airfoil is doubled and all other factors remain unchanged, the lift produced by the airfoil will also double. This is because the circulation is directly proportional to the lift. Therefore, the correct answer is b) Doubles.