The correct answer is option 3 - Triangular. The Indian Peninsular Plateau is shaped like a triangle, with its base running along the southern coast of India and the apex at the central highlands of the Deccan Plateau. The northern boundary of the plateau is defined by the Aravalli Range, while the eastern and western boundaries are marked by the Eastern Ghats and the Western Ghats respectively. The plateau gradually slopes down towards the east and west, resulting in a triangular shape. This shape is evident when looking at a map of India, where the plateau occupies a large portion of the southern and central regions. Therefore, the correct option is triangular.