Researchers believe that the continued maturity of which lobe of the cortex during young adulthood is responsible for cognitive changes such as focusing attention and goal-directed behaviors that lead to expertise:
A. frontal
B. parietal
C. occipital
D. temporal

frontal

Explanation:  the overall volume of the brain do addup (increases) at the earlier(first) life stage and then becomes at a stand still (stable) .adolescence is the level of development where children are becoming adults thereby developing in terms of their intellectual capabilities, physical growth e;g increase in height, hormone activation e.t.c. they have a series of life changing or transformation .adolescence maturing in the brain is usaully between 10–23 years and it is influenced by different factors such heredity and environment, prenatal/postnatal insult, nutrition e.t.c

The frontal lobes of the cortex contains different components of the neural circuitry e.g planning, working memory, impluse control e.t.c. Usuall at times, they are not fully mature or developed later stage of life. .The prefrontal cortex in the simple sense usually help in  higher-order cognitive processes and executive functioning.