Aplant has two alleles for the color of its flowers. w represents the dominant trait, and w represents the recessive trait. identify the homozygous dominant genotype from the test cross.

Capital letters represent dominant traits and the small letters or lowercase letters represent recessive traits. If they say homozygous that means both letters of the genotype should be either both capital or both lower case. 

It says to identify HOMOZYGOUS DOMINANT then that means the genotype would be both capital so your answer would be WW. 

If in case they would ask for HETEROZYGOUS that means one is dominant and one is recessive. So that means it would have one capital and one lowercase. So heterozygous would be Ww. (Just added info)