Which type of muscle adaptation is primarily a result of resistance training?

Muscle hypertrophy is the correct answer because it specifically refers to the increase in the size of muscle fibers resulting from resistance training. When muscles are subjected to the stresses of weight lifting or other forms of resistance, they sustain micro-tears. The body repairs these tears by increasing the size and number of muscle fibers, leading to overall muscle growth. This adaptation is an essential part of the physiological response to strength training, as it helps to enhance performance and strength.

In contrast, aerobic endurance relates more to exercises that improve the efficiency of the cardiovascular system and the muscles' ability to utilize oxygen during prolonged activities, typically associated with endurance training rather than resistance training. Flexibility improvement refers to the range of motion of joints and muscles, which is not a primary focus of resistance training. Muscle atrophy, on the other hand, is the decrease in muscle size, often resulting from a lack of physical activity or resistance training, making it the opposite of hypertrophy. Thus, the primary adaptation resulting from resistance training is indeed muscle hypertrophy.