The relationship between "pen" and "write" is that a pen is a tool used for writing.
Following this logic:
- "lungs" are a body part used for breathing.
- "microscope" is a tool used to magnify.
- "camera" is a tool used to photograph.
From the options given, the pair that shows a similar relationship is:
:: stomach: digest
This is because the stomach is a body part used for digestion, just as lungs are used for breathing.