Which one of the following material will require the largest size of riser for the same size of casting?

Answer: Option C

The correct answer is (C) Steel.
Here's why:

What is a Riser?
A riser is a reservoir of molten metal connected to the casting.
Its main job is to compensate for shrinkage that happens as the metal cools and solidifies.

Why different materials need different sized risers:
Different metals shrink by different amounts when they cool down from liquid to solid.
The greater the shrinkage, the bigger the riser needs to be to feed the casting and prevent voids (holes) from forming inside.

Why Steel needs the biggest riser:
Steel has the highest shrinkage rate among the given options (Aluminum, Cast Iron, Copper).
This means it shrinks the most when it solidifies.

Cast iron actually expands slightly during solidification, making it need the smallest riser.
Aluminum and Copper shrink, but less than steel, so they require risers smaller than steel's.

In short: Because steel shrinks the most as it solidifies, it needs the largest riser to supply enough extra metal to prevent defects in the final casting.