Answer: Two Labrador retrievers with black fur can reproduce puppies with yellow fur only when their genotypes are heterozygous dominant for black fur that is- Bb and Bb.
Exaplanation-
As per the given information in the question, black fur is dominant (B) to yellow fur (b). A dominant trait ( depicted by capital letter) masks the expression of recessive trait.
There can be two possible genotypes for black fur that is BB ( homozygous dominant) and Bb ( heterozygous dominant) whereas yellow fur ( which is a recessive trait) can only have one genotype that bb ( homozygous recessive).
When 2 Labrador retrievers with heterozygous dominant genotype ( Bb) are crossed, they is 25% chance of producing a puppy with yellow fur ( bb).
Refer punnett square.