Explanation

Hazrat Khalid-bin-walid (RA) is widely recognized as the first great commander of the Islamic army. Hazrat Umer (RA) was a prominent leader but not the very first commander in this context. Hazrat Hamza (RA) was a highly respected warrior but not considered the first commander overall. None of them is incorrect because Khalid-bin-walid (RA) fits the description best.