Biologists once assumed that the phenotype of a dominant allele would always be the most common phenotype in a population. which finding led biologists to a different conclusion?

C. the discovery of those populations in which phenotype of recessive allele a population survive. 
if recessive allele population to survive, than it will passed down to the offspring. the dominant allele will be eliminated. hence, in this case the common phenotype will be of that recessive allele. this is an example of directive natural selection/evolution (directed towards the recessive allele).