During solidification of a pure molten metal, the grains in the casting near the mould wall are

The correct option is fine and randomly oriented.

During the solidification of a molten metal in a casting mould, the solidification process typically proceeds through the formation of distinct zones: the chill zone, the columnar zone, and the equiaxed zone.

When the hot molten metal comes into direct contact with the cold mould wall, rapid heat extraction occurs. This sudden drop in temperature at the boundary leads to a high rate of supercooling (or undercooling) in the liquid metal adjacent to the wall.
According to nucleation theory, a high degree of undercooling significantly increases the rate of heterogeneous nucleation. Because many nuclei form simultaneously on the cold surface of the mould, there is limited space for each grain to grow before impinging on neighboring grains.

Consequently, the grains in this region (known as the chill zone) have the following characteristics:
1. Fine size: Due to the extremely high density of nucleation sites, the resulting grains are very small (fine).
2. Random orientation: Since the nuclei form independently and randomly on the rough mould wall surface, their crystallographic orientations are completely random.