If flower color in a particular species of plant is a monogenic trait, and a genetic cross between a plant that is true-breeding for red flowers and one that is true-breeding for white flowers always results in offspring that have flowers containing a mix of BOTH red and white petals, then the red and white alleles for flower color must be:

The red and white alleles for flower color must be codominant.

Explanation:

When two alleles are codominant there isn't a dominant-recessive relationship between alleles. In a heterozygous individual both alleles are equally expressed, and the individual will show both traits at the same time.

A true-breeding plant with red flowers (RR) is crossed with a true breeding plant with white flowers (WW), producing a homogeneous RW offspring. These flowers have both red and white petals, so these alleles are codominant.

A more familiar example of codominant alleles are the A and B alleles that determine blood types. A heterozygous person with both an A and a B allele has an AB genotype (both alleles are expressed at the same time).