(1101 0001)2 binary number is same as which octal number?

The correct answer is A, (321)8. To turn a binary number into octal, group the bits in threes starting from the rightmost bit, adding leading zeros to complete the last group, because three bits cover the eight octal digits. The number 11010001 splits into 11, 010 and 001, and the first group is written as 011. The group 011 has the value 3, the group 010 has the value 2 and the group 001 has the value 1, so the octal number is 321. Checking in decimal confirms it: 11010001 is 128 + 64 + 16 + 1, which equals 209, and octal 321 is 3 times 64 plus 2 times 8 plus 1, again 209. Option B is wrong because 123 is the same digits reversed. Option C is wrong because 641 would need the bits 110100001. Option D is wrong because 146 in octal equals 102 in decimal.