Correct : 6.283

This question comes directly from Prandtl''s Lifting Line Theory, which is one of the most fundamental topics in finite wing aerodynamics. Let''s build up to the answer from first principles so the reasoning is completely clear.
For a finite wing with an elliptic planform, the lift-curve slope is given by:
dC_L/dα = a₀ / (1 + a₀ / (π × AR))
Where a₀ is the two-dimensional (2D) lift-curve slope of the airfoil section, and AR is the aspect ratio of the wing. For a thin airfoil, thin airfoil theory gives a₀ = 2π per radian.
Now, the question asks what happens as AR → ∞. As the aspect ratio becomes infinitely large, the denominator of the formula approaches:
1 + 2π / (π × ∞) = 1 + 0 = 1
So the lift-curve slope becomes:
dC_L/dα = 2π / 1 = 2π ≈ 6.283 per radian