The correct answer is: B. spiral
A spiral is a curve that winds around a central point, getting closer and closer to the point as it goes. It is a type of curve that is often used in architecture and engineering, as it can be used to create smooth transitions between two different shapes.
The shape of transition curve used by Indian Railways is a spiral because it allows trains to change direction smoothly without causing any discomfort to the passengers or damage to the train. The spiral curve is also more efficient than other types of curves, as it allows trains to travel at a higher speed.
The other options are incorrect because they are not types of curves that are used in railways. A cubic parabola is a curve that is shaped like a parabola, but with a cubic equation. A sine curve is a curve that is shaped like a sine wave. A lemniscate of Bernoulli is a curve that is shaped like a figure eight.