Answer:
The most appropriated answer would be "The population evolved because the frequency of the alleles and the genotypes changed".
Evolution is a gradual process by which the inherited characters of a population change over successive generations.
Thus, it results in a change in the alleles frequencies of the population.
It might result from natural selection, genetic variation, gene mutation, genetic drift et cetera.
Similarly, in the given population the allele frequencies of the alleles associated with flower color are changed. Thus, the population must have evolved.