The given sentence expresses a tense in the present time, as suggested by the word 'now'. The person is making a suggestion to perform an action, while another action is taking place (or rather, not taking place).
Here, 'while' means refers to two actions that are taking place at the same time.
Option A: 'Has not rained' refers to an action that began in the past and just completed. But here, the action has not yet happened.
Option B: 'Will not rain' declares that an action will not take place in the future. But, the action must be taking place now, as suggested by 'now, while'.
Option D: 'Had not rained' presents an action that began and completed in the past. But, here, both the actions are in the present.
Hence options A, B and D are incorrect.
Option C: 'Isn't raining', or 'is not raining' indicates a continuous action in the present time. It agrees to the given sentence, as the actions are both in the present time.
Hence option C is correct.