(c) 936
The number of 4-letter words which can be formed from 6 letters when one or more of the letters is repeated = 6 × 6 × 6 × 6 = 1296
Number of 4-letter words which can be formed from the given 6 letters when none of the letters is repeated
= Number of arrangements of 6 letters taken 4 at a time
= 6P4 = \(\frac{6!}{(6-4)!} = \frac{6!}{2!}\) = 6 × 5 × 4 × 3 = 360
∴ Number of 4 letter words which have at least one of their letters repeated = 1296 – 360 = 936.