Final answer:
The most likely risk of passing on an autosomal dominant disease, when both parents are affected, is 50%.
Step-by-step explanation:
For an autosomal dominant disease, if two affected parents have a child, the most likely risk of passing on the disorder is 50%. This is because autosomal dominant diseases only require one copy of the mutated gene for the disorder to be expressed. If both parents are affected, there is a 50% chance that they will pass on the mutated gene to their child.