Aplant with dark-purple flowers and a plant with white flowers are crossed. all of the offspring have lavender flowers. the alleles for flower color in this plant are
a. both dominant
b. both recessive
c. codominant
d. incompletely dominant
answer ? ?

I believe the answer is incomplete dominance. Incomplete dominance creates a mix of the two phenotype(white and dark purple flowers) to create a third phenotype (lavender). Lavender is a mix.

Whereas, co-dominance would not be a mix but both traits would exist within the organism. In your question, it would be codominance if the dark purple flower and the white flower created a flower with white and dark flower pedals. This would not be a mix of the phenotype, but a co-existance of both phenotype.

I hope that helps! :D