Answer:
The purple flower is a dominant trait.
It can be explained with the help of the law of dominance which states that the trait which expresses itself and masks the other in a heterozygous condition is called a dominant trait.
When two purebred parents for a particular trait (like flower color in this case) are crossed, it always results in a heterozygous offspring. The trait which is expressed in the heterozygous condition is said to be a dominant trait.
Like in this case, the offspring had purple flowers. Thus, the purple flower is a dominant trait.