D(√3, 1)
Let AD tilt angle be α, BD tilt angle be β and AB tilt angle be π/6.
The angle between AB and AD is ∠ 1=α-π/6.
The included angle between AB and BD ∠2=π/6+π-β.
We can know ∠ 1=∠2 from the nature of the circle.
∴α-π/6=π/6+π-β==>; α+β=4π/3
Option c