Step 1: What is the greenhouse effect.
The greenhouse effect happens when certain gases trap heat in the air. These gases soak up infrared radiation coming from the warm Earth.
Step 2: What makes a gas trap heat.
A gas traps heat only if its molecules can absorb infrared rays. Molecules with more than one kind of atom often do this well.
Step 3: Look at carbon dioxide.
Carbon dioxide absorbs infrared rays strongly. So it holds heat in the air and is a major greenhouse gas.
Step 4: Look at nitrogen and oxygen.
Nitrogen and oxygen make up most of the air, but they do not absorb infrared rays well. So they do not cause the greenhouse effect.
Step 5: Look at argon.
Argon is a noble gas with single atoms. It does not absorb infrared rays, so it does not trap heat.
Step 6: Final choice.
Only carbon dioxide strongly traps heat, so it causes the greenhouse effect.\[ \boxed{\text{CO}_2} \]