[August]

While one could possibly make a case against Employee A, it's apparent that Employee B has the best qualifications of the lot, at least if you count seniority.

But as folks have pointed out there could be other aspects of this.

First Employee D is a non starter. Not only does he have no qualifications he isn't seriously going for the job. As the hiring manager I might even be inclined to see his interview as an unwelcome waste of my time.

As for the other two there could be personality conflicts. The hiring manager has a right to hire people he can get along with. Compatibility among team members, especially at management levels, can be more important for success than professional qualifications. The last thing a boss needs is conflict in his team.