In this genetic scenario, white coat (W) is dominant over tan coat (w). When a white-coated male (whose genotype could be WW or Ww) is mated with a tan-coated female (whose only possible genotype is ww because the tan coat is recessive), the offspring's phenotypes depend solely on the male's genotype.
Since 100% of the offspring from this mating are white-coated, the male must be homozygous (WW) for the white coat trait. The tan-coated female must be homozygous recessive (ww) because she exhibits the tan coat phenotype.
Therefore, the genotype of the tan-coated female fox is:
D. ww