Assertion-Reason relation :
Syncytium is a condition of multinucleate cells (condition having numerous nuclei). This happens due to aggregation or fusion of cells followed by dissolution of their cell membranes.
Muscle fibres show multinucleate condition, specifically syncytial condition. This is because the muscle cytoplasm, also known as sarcoplasm, contains many nuclei. This is because the development of muscle fibres involves the event of fusion of myoblasts during embryonic development.
So, both the assertion and reason are correct. Also the reason is the correct explanation of the assertion.