If you are only given glycogen and ATP as choices, then glycogen is the correct answer. The reason for this is, that muscle cells need ATP also for other very important processes (such as keeping the membrane potential stable and preventing cramps). Thus, while ATP is the actual fuel that powers myosin to create the muscle force, the cell needs to keep the ATP concentration constant in order to avoid negative impacts on other metabolic processes. Therefore glycogen is the actual energy storage.
However glycogen is not the only energy storage used in muscles. The muscle actually uses a quite clever energy management system:
During the first 2-7 seconds it uses phosphocreatine (or creatine phosphate) to quickly replace used ATP (as mentioned in the answer by <PERSON>). 171
This means a 100m Sprint can be almost completely performed using this energy storage system.
For the next ~30 min (but can be up to several hours for well trained endurance athletes), the muscle relies mainly on energy stored as glycogen. The glycogen is broken down into glucose and the glucose is then converted into glucose-6-phosphate and then ATP during glycolysis. Glycolysis creates ATP much faster than oxygen respiration. However, a side-effect of this is that glycolysis generates lactic acid which is further broken down by oxygen respiration, and in the liver. Overtraining can cause lactic acid accumulation whcih can acidify the muscle and cause lactic acidose There is much more energy stored in glycogen than in phosphocreatine which also makes this the more likely answer to your question.
Finally, after glycogen stores are depleted, the muscle needs to reduce its energy output and rely on external energy provided by the liver and by fat reserves of the body. This is the reason why some trainers claim that you need to exercise for more than 30 min to start burning fat and thus to reduce weight but this is wrong because the glycogen stores in the muscles are also restored using energy from body fat (which means that the best and only way to loose weight is to burn more energy than you consume either by increasing exercise or reducing food intake or both). 860