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 pure molten metal in a casting mold, the process of grain formation occurs in distinct zones depending on the rate of cooling and heat transfer:

1. Rapid Cooling (Chill Zone): When the hot liquid metal comes into contact with the relatively cold mold wall, rapid heat extraction occurs. This extremely high cooling rate triggers rapid nucleation of solid crystals at the surface of the mold wall.
2. Grain Refinement: Because the cooling is very rapid and there are numerous nucleation sites on the mold surface, a large number of crystals start growing simultaneously. Since they grow close to one another, they quickly obstruct each other's growth, resulting in a layer of very small or fine grains near the mold wall (known as the chill zone).
3. Random Orientation: Nucleation occurs randomly on the mold surface. Because of the initial rapid rate of solidification, these fine grains do not have sufficient time or directionality to align themselves in any preferred direction, making them randomly oriented.

As solidification progresses further inward, heat transfer becomes directional (normal to the mold walls), leading to the growth of column-like, ordered grains (columnar zone), but the grains immediately near the mold wall remain fine and randomly oriented.