Step 1: Understand the task.
We must find the pollutant whose effect is matched wrongly. We check each pair against known facts.
Step 2: Check CO.
Carbon monoxide binds haemoglobin and starves the body of oxygen, so option 1 is correctly matched.
Step 3: Check hydrocarbons.
Hydrocarbons harm plants and speed up plant ageing, so option 2 is correctly matched.
Step 4: Check $CO_2$.
Carbon dioxide is a greenhouse gas that drives global warming, so option 4 is correctly matched.
Step 5: Check the fluoride statement.
Fluoride up to about $1$ ppm is actually good for teeth; it only becomes harmful to bones at higher levels (above about $2$ ppm). So saying $F^- < 2$ ppm is harmful to bones is wrong.
Step 6: Conclusion.
The incorrectly matched pair is $F^- < 2$ ppm with harm to bones. \[ \boxed{F^- < 2\ \text{ppm} \ne \text{harmful to bones}} \]