Absolute value is the distance from 0, and we always measure distance using positive numbers. Even if I back my car up 20 feet, I drove 20 feet and not -20 feet, even though I'm 20 feet behind where I started, or -20 on a number line. If I drove forward 20 feet to 20 on a number line, I still drove 20 feet. This is why -20 and 20 have an absolute value of 20, because they are both 20 feet from 0.
A negative shows a loss in value, but the loss is still $15.75 because it's $15.75 away from it's value the previous day, or 0 on a number line.
Your answer is a.