The type of adaptation occurring in this scenario is hypertrophy. Hypertrophy refers to the increase in muscle size or bulk due to repetitive and progressive resistance training. When a bodybuilder repetitively lifts weights, the biceps muscle is subjected to mechanical stress and microtrauma. This stress triggers a cascade of physiological responses, including an increase in protein synthesis within muscle fibers. Over time, the muscle adapts by increasing the size and number of myofibrils, leading to the enlargement of the biceps muscle. This adaptation is known as hypertrophy and is a common goal for bodybuilders and strength trainers.