In a casting process, a vertical channel through which molten metal flows downward from pouring basin to runner for reaching the mold cavity is called

The correct option is sprue.

Step-by-step Explanation:

To understand why a sprue is the correct answer, let's break down the components of a typical gating system used in sand casting:

1. Pouring Basin: This is the reservoir at the top of the mold where the molten metal is initially poured from the ladle. It helps reduce turbulence and regulates the initial flow of the metal.
2. Sprue: The molten metal flows downward from the pouring basin through a vertical channel. This vertical passage is called the sprue (often tapered to prevent air aspiration and maintain continuous flow). Its primary function is to feed the metal from the pouring basin down to the runner.
3. Runner: The runner is a horizontal channel that receives the molten metal from the bottom of the sprue and distributes it to the gate(s) leading into the mold cavity.
4. Riser: A riser is a reservoir of molten metal built into the mold to feed liquid metal to the casting as it shrinks during solidification.

Now let's examine the options provided in the question:
• blister: A defect on the surface of a casting consisting of a hollow bubble or pocket of gas. It is not a gating system channel.
• sprue: As defined above, this is the vertical channel that connects the pouring basin to the runner.
• riser: A reservoir designed to supply feeding metal during solidification to prevent shrinkage cavities. It does not channel the metal downwards from the pouring basin to the runner.
• pin hole: A casting defect characterized by numerous tiny holes on or near the casting surface, caused by entrapped gases.

Therefore, the vertical channel through which molten metal flows downward from the pouring basin to the runner is called a sprue.