A cross between two snapdragon varieties - one with red flowers and one with white flowers produces $F_1$ hybrids all with pink flowers . This is an example for

The correct option is D

Incomplete dominance

Not all alternative alleles are fully dominant or fully recessive in heterozygotes. Some pairs of alleles instead produce a heterozygous phenotype that is either intermediate between those of the parents (incomplete dominance), or representative of both parental phenotypes (codominance). In snapdragon, crossing pure red (RR) and pure white (rr) flowered plants produces pink (Rr) flowered plants. In this case the dominant allele ‘R’ is not completely dominant over the recessive allele ‘r’. This phenomenon is called incomplete dominance.