Correct option is (d) 999
The first ring can be marked with any of the 10 numbers, i.e., Number of ways of marking the first ring = 10
Similarly the second and third ring can also be marked with any of the 10 numbers.
∴ Number of number combinations that can appear on the 3 rings = 10 × 10 × 10 = 1000
The lock can be opened with a single combination only.
∴ Number of cases in which the lock cannot be opened = 1000 – 1 = 999.
