Question

why can an organism with a heterozygous genotype (hh) express the dominant trait (tall) but not the recessive one (short)?
because the recessive allele is stronger than the dominant allele.
because the dominant allele masks the effect of the recessive allele.
because two recessive alleles are needed for the trait to be expressed.
because the environment changes the expression of the gene.

Explanation:

Dominant alleles exhibit their phenotypic effect even in the presence of a single copy (in heterozygotes) by masking the expression of the recessive allele, which only shows its effect when two copies are present. The idea of "strength" between alleles is incorrect, and environment is not relevant to this core genetic dominance mechanism here.

Answer:

Because the dominant allele masks the effect of the recessive allele.