How many different phenotypes are possible in the offspring of two parents with genotypes aa and aa, where a is the dominant allele for a trait and a is the recessive allele for the same trait?
one
two
three
four

The answer is one.

Genotype is the entire set of genes in the organism. Phenotype is the physical characteristics of an organism which are the result of interaction between genotype and environment.
Allele A is dominant over allele a. We have three possible genotypes - AA, Aa, or aa, and two different phenotypes: phenotype with dominant trait (genotypes AA and Aa are responsible for this phenotype) or phenotype with recessive trait (genotype aa is responsible for this phenotype).
So, we cross two parents with genotypes AA and aa. Each of them can give only one type of alleles (AA can give only A allele, aa can give only a allele) to the offspring. So, all of the offspring will be with genotype Aa and with phenotype with dominant trait (take a look at the attached image).