Regardless the answer to the question; describe how natural selection ensures the most favourable alleles are passed on
Random mutations within the species causes changes to the phenotypes of the organisms the alleles that causes these changes may provide an advantage in survival
A selection pressure is applied which causes organisms who are better adapted to survive.
Organisms that are able to survive have a higher chance of producing offspring - note that there is a trade off i.e an organism needs to be able to survive the selection pressure but equally the organism still needs to attract a mate it is no good being well adapted to survive but not being able to reproduce.
Fertile offspring have a high chance of inheriting these advantage alleles (however do note there is a slim chance that they will not inherit these alleles due to variation)