In a recent report, the World Nuclear Association stated that in over six decades of nuclear power, only three major accidents have ever occurred among the more than 400 nuclear power plants currently operational worldwide,
but that with advances in safety technology, the probability of future accidents continues to decline.
A) but that with advances in safety technology, the probability of future accidents continues to decline -> We don't have contrast in sentence. So, "but" is incorrect to use.
B) and with advances in safety technology, the probability of future accidents continue to decline down -> "the probability ...... continue" is incorrect. It has a SVA error.
C) but with advances in safety technology, the probability of future accidents continues to decline -> Same error as A. Incorrect.
D) and that with advances in safety technology, the probability of future accidents continues to decline -> SVA error is corrected. We see a parallelism as well, "that in over six decades of nuclear..." and "that with advances in safety technology...". Let's keep it.
E) so that with advances in safety technology, the probability of future accidents continue to decline -> Same error as B. Incorrect.
So, I think D.