Pay close attention to the word "purposes." A manual that explains what jumpers and switches are for is different from a manual that tells you how to configure them correctly for your specific setup. The obstacle the author describes is not that customers don't understand what jumpers/switches do or why we use them, but that the specifications for how to set them are "mysterious." Knowing the purpose of a jumper/switch doesn't tell you which setting satisfies the requirements of your particular combination of accessories. So (A) doesn't address the actual problem the author identifies.

Even if the software is free, the author's concern is not about cost. The problem is that adding software for one accessory can disable another accessory. Whether that software is free or expensive doesn't change whether installing it causes conflicts.

This describes what may happen in the future, but the author's argument is about the current state of affairs. Whether installation will become easy later has no bearing on whether complexity is a widespread obstacle to use right now.

The author's evidence focuses on customers who install accessories themselves. But if a personal computer is typically sold as a package that already includes accessories and comes with free installation, then this is evidence that the group of customers who have to personally navigate jumpers, switches, and conflicting software is not actually widespread. Even if the premise may accurately describe what self-installation is like, it wouldn't support the conclusion that complexity is a widespread obstacle if most customers are never in that situation to begin with.

This, if anything, supports the author's argument. If manufacturers don't take into account ease of installation when designing their computer products, that's more reason to think users might face difficulties during installation.