Final answer:
The weakened magnetic field of Mars allowed for increased solar radiation that stripped away the atmosphere, leading to a cooler, drier climate with less capacity to retain heat.
Step-by-step explanation:
The question concerns how Mars' weakened magnetic field might have led to major changes in its climate. The correct answer is a) Increased solar radiation. Mars' diminished magnetic field allowed for more solar radiation to reach the planet's surface, which had dramatic effects on its atmosphere and climate. Without a protective magnetic field, the atmosphere became vulnerable to the solar wind, a stream of charged particles emanating from the Sun. This led to the gradual loss of Mars' atmosphere as particles stripped away atmospheric gases into space. Consequently, the loss of atmospheric pressure reduced the greenhouse effect, leading to a cooler climate and the freezing of water. This runaway refrigerator effect contrasts with the runaway greenhouse effect on Venus, leading to the cold, dry conditions on Mars we see today.