Option 1 -> Coal is somewhat brittle but not the standard comparison for this simile.
Option 2 -> Glass is extremely brittle and shatters easily, making it the classic comparison for brittleness.
Option 3 -> Iron is a strong, tough metal that doesn't break easily, so it's not brittle.
Option 4 -> Stone is hard and solid but not commonly associated with brittleness.
Hence, Option 2: glass -> "As brittle as glass" is a well-established simile in English that describes something fragile and easily broken. Glass is universally recognized for its brittle nature, making it the perfect comparison for this expression. -> correct