Answer: A

Given that a - b = c - d, a fraction such as (a - b)/(c - d) or (c - d)/(a - b) would be equal to 1. However, if one of the halves of the fraction were multiplied by -1, the result would be -1. For instance, (b - a)/(c - d), or (d - c)/(a - b) would be equal to -1.

Neither of those examples are among the choices, so you'll have to work harder. Try rearranging the equation for each choice. For instance, since the numerator of (A) is a - c, arrange the equation so that a and c are on one side, and b and d are on the other:

a - c = b - d

Thus, (a - c)/(b - d) is equal to 1, meaning that if we multiply the denominator by -1, for a result of (a - c)/(d - b), the fraction is equal to -1. This is the correct choice.

If you want to test the other choices, work through the same process.