Option D4: Hardy-Weinberg Principle

Hardy Weinburg Equation
Requires: a dominant and a recessive allele for a gene, only traits that are autosomal

Since there are only two alleles for the gene, there can only be three genotypes: AA, Aa and aa.
Dominant alleles have a frequency of p, recessive alleles have a frequency of q
So that the total frequency of the alleles must be 1 (probabilities in math) p + q = 1
Genotypes have two alleles so the equation can be squred to give the Hardy-Weinberg equation: 
              p² + 2pq + q² = 1            

The Hardy Weinburg equation is usually used under the assumption that the allele frequency will remain over time. It is implied that the population is large, there is no random mating, mutation, gene flow (immigration/emigration) or natural selection.