Final answer:
DNA wraps around histone proteins to form nucleosomes, which then coil into chromatin fibers. These fibers further condense to create chromosomes, thus the order from least to most complex is DNA, nucleosome, chromatin, chromosome.
Step-by-step explanation:
To determine the correct order of structures from least to most complex organization, we should understand how DNA is packed within the cell nucleus. The first level of organization, or packing, is the winding of DNA strands around histone proteins to form structures called nucleosomes. This looks like beads on a string under the electron microscope. Multiple nucleosomes coil together to form a fiber known as chromatin, which further condenses to form a chromosome.
Based on this information, the correct order from least to most complex is:
DNA
Nucleosome
Chromatin
Chromosome
Thus, the answer to the question is option (a) DNA, nucleosome, chromatin, chromosome.