The question asks which of the given options corresponds to the octal representation of the binary number "0001 0001."

Let's break down the binary number and convert it to octal:

1. Binary Number: $0001\ 0001$

2. Separate into groups of three bits from the right for octal conversion: $000\ 100\ 001$.

   Since octal requires groups of three bits, let's adjust accordingly:

   • $000$ is already a complete group.
   • The next three bits are $100$.
   • The last group is $001$.

3. Convert each group to octal:

   • $000$ in binary is $0$ in octal.
   • $100$ in binary is $4$ in octal.
   • $001$ in binary is $1$ in octal.

4. Combine the octal digits to get the result: $021$.

Therefore, the correct answer is:

d) 21

Would you like a detailed explanation of the binary-to-octal conversion process?

Here are some related questions you might find interesting:

1. How do you convert a binary number to decimal?
2. What are the rules for converting binary numbers to hexadecimal?
3. How does binary addition work?
4. What is the octal system, and how is it used in computing?
5. Why is binary important in computer science?

Tip: When converting binary numbers to octal, group the binary digits in sets of three from the right, adding leading zeros if necessary to complete the groups.