Runoff is a flow over of water or any other liquid over a surface causing erosion. The flow of water is of high speed and rapid. It is not absorbed by the soil.
Plant grass and lay straw at the sites is the correct option this is because of the fact that the grass and lay straw will absorb the runoff water and prevent the erosion.
It is given that the allele B for the black coat is dominant over allele b for a brown coat.
A dominant allele always leads to the expression of the dominant trait in the heterozygous condition.
When a black and brown horse is crossed as given in the question and the result is a brown foal, then the genotype of the brown foal must be bb as the brown coat is a recessive trait.
Since each parent contributes one allele each for a given gene, the black horse must have b allele, and hence must be heterozygous Bb.