D. He wasn't a complete failure, as he didn't outright cost us any games, and he redeemed himself with what appeared to be some better field awareness toward the end of the season (INT comes to mind) but it's absolutely clear that he is not a safety. He's a man-cover, backup cornerback.
The fundamentals just aren't there. The number one thing a safety cannot do (other than fail to continue to backpedal, which Ball did on numerous occasions) is get caught staring into the backfield. He did this countless times. Goal line defense was horrific...as a FS playing near the goal line, you plant your heels on the goal line and play from there -- too often he got caught staring into the backfield and getting sucked up near the line of scrimmage when he was supposed to be patrolling the end zone.
It's also clear that he was afraid to make tackles on larger players (mostly tight ends, not RBs), one of the things that many people were worried about.
All in all, like I said, he was bad. But he wasn't failing bad. An addition of Daniel Manning or Michael Huff would go great lengths to fixing this situation. Ball might find better opportunities to play and get paid in the UFL this upcoming season.