The first option is the correct one. Since the disorder is dominant, a normal parent must be a recessive homozygote, let's say -dd-. If this parent crosses with a heterozygote parent, that is -Dd-, the chances that the newborn will inherit the disorder are 1/4. We can see that the normal parent only gives recessive alleles -d-, that will account for 50% of the probability. In the other case, the parent could provide, either a disorder allele -D- or a recessive allele -d-, so this parent adds the only chance to inherit the disorder, that is 25%.