Assume that a cross is made between AaBb and aabb plants and that the offspring fall into approximately equal numbers of the following groups: AaBb, Aabb, aaBb, aabb. These results are consistent with the following circumstance:

alternation of generations
independent assortment
incomplete dominance
complete linkage

Independent assortment.

Explanation:

Law of independent assortment is one of the laws that Mendel proposed in genetics. This law states that the alleles of two or more different genes get sorted into gametes independent of each other i.e. the alleles of two different genes do not influence each other at the time of gametogenesis.

For example, in this question when parents having allelic combinations AaBb and aabb are crossed they produced offsprings with allelic combinations AaBb, Aabb, aaBb and aabb. It simply indicates that the alleles of traits controlled by alleles Aa & Bb are sorting into any of the 4 gametes which are being produced. If they were not assorting independently then gametes with Aabb and aaBb combinations which are different than parents would not have produced.

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.