The correct answer is option 3, which suggests the occurrence of ice caps and frozen water as the most relevant condition for the possibility of life on Mars. The main reason this is so crucial is because water is one of the prerequisites for life - or at least the form of life as we know it - since all living organisms need water in some capacity to survive and thrive.

Taking into account the other options, option 1 refers to the atmospheric composition. Whilst this is indeed important, life is capable of adapting to varying atmospheric compositions, and as such, it is not considered as essential as the presence of water. Option 2 mentions thermal conditions. While temperature does affect life, extremophiles (organisms that thrive in extreme environment) on Earth have shown that life can exist in a broad range of temperatures. Finally, option 4 mentions the occurrence of ozone. Although ozone is important for blocking harmful solar radiations, it`s not as central to sustaining life as the presence of water.