The correct answer is C. A buildup of lactic acid in the tissues.
Sore muscles after vigorous exercise are the result of lactic acid accumulation in the muscles.
Vigorous exercise reduces the levels of oxygen available in the muscles due to which complete oxidation of the glucose could not take place.
Muscle cells switch to another process called as lactic acid fermentation to produce energy. In this process, lactate dehydrogenase converts pyruvate into lactate and reduces NADH to NAD⁺.
This NAD⁺ enables the continuation of glycolysis which results in the production of net 2 ATP.
In addition, influx of materials such as nutrients, WBC, anti-inflammatory compounds etc into the muscle cell (for repair) causes swelling of the muscle fibers which is also the reason for the muscle soreness