Final answer:
The ice caps on Mars are made of frozen CO₂ (dry ice) and water ice. They change during the seasons, growing during the summer.
Step-by-step explanation:
The ice caps on Mars are made of frozen CO₂ (dry ice), not ordinary snow. They condense directly from the atmosphere when the surface temperature drops below about 150 K. These caps develop during the cold martian winters and extend down to about 50° latitude by the start of spring.
The northern permanent cap, on the other hand, is much larger and is composed of water ice. It never shrinks to a diameter less than 1000 kilometers. The temperature in the north is too high for frozen CO₂ to be retained, so the cap is predominantly made of water ice.
Therefore, the correct answer is b) Made of water ice, they grow during summer.