Think about what changes, and what does not change, when petrol is made unleaded.
Option A, carbon monoxide: this does form during incomplete combustion, but it is a distinct pollutant, its presence is not what this option set is highlighting as the key emission of unleaded petrol.
Option B, carbon dioxide: this is the product of complete, efficient combustion of the hydrocarbon fuel, its formation has nothing to do with whether the petrol had lead in it or not, so it is not the target answer here.
Option C, ethylene: this is not a routinely cited standard component of petrol engine exhaust in this kind of question, so it does not fit.
Option D, hydrocarbons: since engines never burn fuel with one hundred percent efficiency, some of the petrol itself escapes the cylinder as unburnt or partly burnt hydrocarbon vapour. Taking the lead additive out of the fuel formulation does nothing to fix this incomplete combustion, so unburnt hydrocarbons continue to appear in the exhaust of unleaded petrol vehicles just as before delead, this is exactly why hydrocarbons is flagged as present in unleaded petrol emissions.
Matching the expected classification, option D, hydrocarbons, is correct.