Final answer:
Plant cells have more than two complete sets of chromosomes to provide redundancy for genetic stability. Polyploidy is common in the plant kingdom and results in larger and more robust plants. Increased genetic diversity is a benefit, but not the primary reason.
Step-by-step explanation:
Plant cells have more than two complete sets of chromosomes because it provides redundancy for genetic stability. Polyploidy, which is the condition of having more than the normal number of chromosome sets, is common in the plant kingdom.
Polyploid plants tend to be larger and more robust than individuals with the normal number of chromosomes. Increased genetic diversity, as mentioned in one of the options, does enhance adaptability in plants, but it is not the primary reason for having more than two complete sets of chromosomes.