If a climate scientist says, "The CO2 concentration in 2018 was 407.4 ppm." What
do they mean? *

Ppm means the concentration of vapors or gases expressed in parts per million of polluted air.

Explanation:

In Chemistry and Physics, part per million or PPM is a measure that expresses the number of units of a certain existing substance for every million units of the mixture. (Concentration)

In climate the equation is used so:

This is used to find the concentration of gases expressed as parts of gas per million parts of the

 polluted air.

In this case, it would be 407.4L of CO2 per 10∧6L of air.

c) should be the answer.