The smallest 7 digit number is
$
  (a){\text{ 1000000}} \\
  {\text{(b) 1 + greatest 6 digit number}} \\
  {\text{(c) either A or B}} \\
  (d){\text{ none of the above}} \\
 $

Hint – In this question we have to find the smallest 7 digits number so think of the largest 6 digits number as the smallest 7 digit number will be just 1 digit more than the largest 6 digit number. Use this concept to get the answer.

Complete Step-by-Step solution:
The smallest 7 digit number is the successor of the greatest 6 digit number.
As we know the greatest six digit number is made up by the greatest digit among all the digits from (0 to 9).
Therefore the greatest six digit number is (999,999).
As we know that the successor is the number which comes immediately after a particular number (i.e. in a particular number add one)
So the successor of the greatest six digit number is (1 + greatest 6 digit number).
So, this is the required answer.
Or in other words we can say the smallest 7 digit number is
$ \Rightarrow 999,999 + 1 = 1,000,000$
So, this is also the required answer.
Hence option (C) (either A or B) is correct.

Note – Whenever we face such type of problems the key concept is to get the largest one digits smallest number than the required number itself as if we can get that then the required number can easily be obtained by performing some mathematical operation of fundamental addition to get the answer. This concept will help you get on the right track to reach the answer.