Dog breeders have a purebred black male dog and a purebred white female dog of the same breed. when they bred these two dogs, all the f1 (first generation) offspring were black. which term best describes the allele for blackness in this dog breed?
a. heterozygous
b. homozygous
c. recessive
d. dominant

D. Dominant

It is dominant as...
The white coat alleles are recessive, meaning the black coat would most likely be dominant alleles. Assuming the black coated dog had the alleles of BB, and the white coated dog had the alleles ww, there would be a 100% chance that the offspring would have Bw alleles, where the black coat is dominant, "all the offspring were black". 
B = Black, W = White
      B    B
w   Bw  Bw
w   Bw  Bw
The diagram above shows that the offspring would be 100% black coated.

Not sure if you wanted an explanation, but I tried.