The correct answer is: will fall.
The temperature was 25 degrees and now it is only 17. This means that the temperature has already fallen by 8 degrees. Based on this information, it is reasonable to predict that the temperature will continue to fall.
Option A, “is falling”, is incorrect because it is in the present tense. The temperature has already fallen, so it is not currently falling.
Option B, “will fall”, is correct because it is in the future tense. The temperature is expected to continue to fall, so it is a prediction about the future.
Option C, “has fallen”, is incorrect because it is in the past tense. The temperature has already fallen, so it is not in the past.
Option D, “had fallen”, is incorrect because it is in the past perfect tense. The temperature has already fallen, so it is not in the past perfect.