The correct answer is: B. machine printed numbers.
OCR stands for Optical Character Recognition. It is a technology that converts images of text into machine-readable text. OCR units are devices that perform OCR.
The least expensive OCR units can read machine printed numbers. This is because machine printed numbers are typically very uniform and easy to recognize. Handwritten numbers, on the other hand, can be much more difficult to recognize, as they can vary greatly in style and appearance.
OCR units that can read handwriting are typically more expensive than those that can only read machine printed numbers. This is because handwriting recognition is a more complex task.
OCR units that can read marks are typically even more expensive than those that can read handwriting. This is because marks can be very diverse and difficult to recognize.
Therefore, the least expensive OCR units can read machine printed numbers.