Number containing three digits ,repitition are allowed ,

what will be the answer ,please explain it.
a-720
b-900
c-999 d-1000

If you mean how many numbers there are that contain 3 digits does 1 count?
If not, the numbers start at 100 and finish at 999 and there are 900 of these,
so answer is b.

the number containing three digits,
repitition are allowed, is

1000 -(D)

since the number started from
0, 1, 2, 3, 4, 5,6, 7,8,9
then, number of ways arrange this 10
digit number taken three at a time
which repitition are allowed
10 x 10 x10 =1000

answer is A.

it alone contains three different digits and so is unique,

d-1000

000, 001, 002, ..., 999