In the equation y=-x4 , what is the unit rate?

The unit rate is -1. because the unit rate has to have a denominator of 1.

-- "three non-zero digits";
so the last one can't be zero

-- "digits cannot be repeated" and "the first two digits are 9 and 8";
so the last one can't be 9 or 8

-- What's left ?
1,  2,  3,  4,  5,  6,  or 7 . . . . . . . Seven possibilities.

-- Probability = (number of possible successes) / (number of possibilities)

That's  1/7 .  (choice-D)