It’s the responsibility of the person who wants to merge.
Obviously any changes that would have caused conflicts would have been reviewed before being merged, and if a change caused an unreasonable number of conflicts it would have been rejected.
Obviously A doesn’t merge into the main branch and then resolved conflicts. A merges the main branch into his development branch, resolves conflicts, and makes sure his branch works. At that point A has a branch that can be merged without conflicts into the main branch. The resulting code is reviewed then merged. If it is a complex change where the review takes long and has the risk of new conflicts appearing, A can ask his colleagues to hold of merging changes until A’s merge is done.