I actually don't dislike option a): it was convenient because it was in the middle of his empire, so all parts were well connected to it. But the option is not very specific.
I would go with the enhanced trade: it has access both to sea and land trade routes: it was also convenient for this reason.
But actually, the exact reason is still debated: for example, it was also because it was easier defended, in a safer region than Rome, and closer to the Asian part of the Empire, which was neglected till then.