What is the actual storage form of energy in muscles? ATP or Glycogen?
This is a typical MCQ which in order to answer you have to have been at somebody’s lectures or be able to read his mind. The antithesis of education! Cells don’t store energy like car batteries. 

Glycogen in muscle is a store of carbohydrate. It can be used to generate ATP if it is broken down to glucose and the glucose glycolysed. I wouldn’t have chosen this as an answer.

The concentration of free ATP in the muscle cell is not sufficient for it to act as store of a high group-transfer potential compound (which is presumably what the question means by energy). This is definitely not the right answer.

However if creatine phosphate (a.k.a. phosphocreatine) was a choice on your MCQ, that was probably the answer expected as it is used in muscle as a storage molecule which can be rapidly converted by creatine kinase creatine and ATP.

PS

As you say in your comment that the only choices were ATP or glycogen, then glycogen is what is expected.