To solve this problem, we need to identify what Dobson units measure.$\newline$Dobson units are a measure of the total amount of ozone in a column of the atmosphere.$\newline$Let's analyze each option:$\newline \bullet$Option 1: CFCs - Chlorofluorocarbons (CFCs) are compounds that contribute to ozone depletion, but Dobson units do not measure CFCs.$\newline \bullet$Option 2: Stratosphere - The stratosphere is a layer of the atmosphere, but Dobson units do not measure its thickness.$\newline \bullet$Option 3: Ozone - Dobson units are specifically used to measure the thickness of the ozone layer.$\newline \bullet$Option 4: Troposphere - The troposphere is the lowest layer of the atmosphere, but Dobson units do not measure its thickness.$\newline$Therefore, the correct answer is Option 3: Ozone.