Disadvantage of using Drosophila for breeding experiment
All options given have advantages but become disadvantageous depending on experiment you are trying to do. Here question is asked for "breeding experiment". In these experiments it is very important to keep track of genetics. If females mates with males of some undesired genotype, it will ruin your experiment objective. 

As option (c) is 'mating soon after emergence of flies' , females and males from same vial (which is generally maintained as single genotype) will start mating soon. Generally this time period is 4 hours. Drosophila melanogaster starts mating after 4 hours at room temperature. This is the precise reason in Drosophila, virgin collection is done within 4 hours of eclosion. 

Imagine you want to set up cross between wild type and some mutant. You will grow them initially in separate vials. Now for crossing , assume you need females from wild type and males from mutant. In this step it is important that females should be virgin else it will be difficult to track your genotype later on. Now if Drosophila starts mating soon after emergence of flies, it is disadvantage for your breeding experiments. 

Note: There is sperm competition but that is special case