Answer:

d.4

Step-by-step explanation:

Using the same argument we can fill up the Units digit in 4 ways. Hence we can form (6)(5)(4) = 120 numbers of three digits using the digits 1,2,3,4,5,6 without repetition. There are 6 choices for the first digit, 5 for the second, and 4 for the third, so there are 6x5x4=120 choices of digits.

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