How many three digit numbers are there in all?
(a) 100
(b) 101
(c) 99
(d) None of these

As we know, there are 10 digits i.e.,$0,1,2,3,4,5,6,7,8,9.$
We can not use '0' at the hundred's place.
So, we can use only 9 digits at the hundred's place.
Also, we can use 10 digits at the remaining two places.
So, total numbers of 3-digit numbers$=9\times 10\times 10=900$
The total number of 3 digit numbers $=900$

Hence, the correct answer is option (D).