Why is Mars called the Red Planet?

Answer: Iron oxide covers its surface

Iron oxide covers its surface ✓ — Correct! Mars is covered in iron oxide, commonly known as rust. Billions of years ago, iron in Mars' rocks reacted with oxygen and water, creating a rusty dust that covers much of the planet's surface. This reddish-brown dust gives Mars its distinctive red appearance. Strong winds blow this dust around, coating everything and even creating the reddish sky we see in photos from Mars rovers.

Its atmosphere is red — Wrong. Mars' atmosphere is very thin (only 1% of Earth's) and mostly carbon dioxide, which is colorless. The sky can appear reddish due to red dust suspended in the atmosphere, but the atmosphere itself isn't red. The planet's red color comes from iron oxide on the surface.

It reflects red starlight — Wrong. Planets don't reflect starlight in a way that would change their color - they reflect sunlight. Mars appears red because its surface is covered in iron oxide (rust), which reflects more red wavelengths of sunlight than blue or green ones.