The correct answer is: A. spiral
A spiral transition curve is a type of curve that is used to connect two different radii of curvature in a highway. It is designed to provide a smooth and gradual change in the direction of travel for drivers, and to help prevent accidents.
A lemniscate transition curve is another type of curve that is sometimes used in highway design. It is similar to a spiral curve, but it has a more complex shape. Lemniscate curves are not as common as spiral curves, but they can be used in situations where a more gradual change in curvature is desired.
A cubic parabola is a type of curve that is not typically used in highway design. It is a relatively simple curve, but it does not provide a smooth and gradual change in curvature. Cubic parabolas are sometimes used in other types of engineering applications, such as the design of bridges.
In conclusion, the correct answer to the question “The transition curve used in the horizontal alignment of highways as per IRC recommendations is” is A. spiral.