In the ASCII character set, the binary representation of the number 90 is 0101 1010. To find the binary representation of the number 91, you simply need to add 1 to the binary representation of 90.
Adding 1 to 0101 1010:
0101 1010
+ 0000 0001
-----------
0101 1011
Therefore, the binary representation of the number 91 is 0101 1011.
So the correct response is:
0101 1011