Alocker combination has three nonzero digits, and digits cannot be repeated. the first two digits are 9 and 8. what is the probability that the third digit is 7?

-- "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)