Which is a run-on sentence? Two hydrogen atoms and one oxygen atom compose
each molecule of water.
Halocarbons contribute to the loss of ozone in Earth's
atmosphere, so many countries have banned them.
A vertical garden, or green wall, is a wall covered
with vegetation, vertical gardens are gaining
popularity around the world.

The last sentence, “A vertical garden, or green wall, is a wall covered
with vegetation, vertical gardens are gaining
popularity around the world” is a run-on sentence.

There is a comma splice between “vegetation” and “vertical,” which makes the sentence a run-on sentence. To fix it, we could put a semicolon in place of the comma, and the sentence would not be a run-on anymore.

Where is the passage?

Number 2 the key is a bleak reminder