History: Which of the following is not a common bit depth?

Which of the following is not a common bit depth?

• 32-bit depth. 0 to 4,294,967,295.
  This is the third most common bit depth.
• 24-bit depth. 0 to 16,777,215.
  This is the second most common bit depth.
• 16-bit depth. 0 to 65,535.
  This is the most common bit depth.
• 8-bit depth. 0 to 255.
  8-bit depth is almost never used.