I would say Nucleic Acids
The correct answer is option C, that is, behavioral isolation.
The phenomenon of behavioral isolation takes place when two populations possess the tendency of interbreeding, but exhibit distinctions in courtship rituals or other kinds of behavior. For example, the western and eastern meadowlarks are very identical birds whose habitat overlap in the middle of the United States.
However, the members of the two species will not mate with each other, possibly as they use different songs in order to fascinate males. The eastern meadowlarks will not respond to the songs of western meadowlarks, and vice versa.
Using a slow- and fast-growing variant of bamboo, Wei and colleagues looked at cell division, growth, and gene expression (through transcriptomics, which measures all the genes being expressed by an individual) to discover which genes may be responsible for fast growth in bamboo. They found that the slow-growing variant had reduced expression of genes relating to cell wall construction, the plant hormone auxin (important for cell growth and cell division), and had irregular cell growth and cell walls. Wei and colleagues suggest that a reduced ability to produce and perceive auxin, combined with a weakened cell wall, are responsible for the slow growth seen in the bamboo variant.
