Correct Answer - Option 3 : Spiral angle/3

__Concepts:__

The deflection angle is the angle from the tangent to any point on the curve. The deflection angle is very nearly equal to** one-third of the spiral angle.**

The angle between the initial tangent and common tangent at the junction of transition curve and circular curve is called **spiral angle.**

__Note:__

A curve of varying radius is known as transition curve. The radius of such curve varies from infinity to certain fixed value. A transition curve is provided on both ends of the circular curve. The transition curve is also called as spiral or easement curve.