What observation is explained by the law of indepent assortment?

Well, independent assortment explains why there is so much genetic variety. Mendel has proven that the alleles of certain genes can be separated independently, meaning that the characteristic will not be solely determined by another allele. This is why there is so much variety and what the law of independent assortment explains. Genetic variety will also come from crossing over in prophase 1, in meiosis.

So in your case, the answer would be Mendel's law of independent assortment