Explanation

B is correct because Hazrat Usamah bin Zaid (R.A) was appointed to lead the first army/Sariya. A is incorrect because Ubaidah bin Haris (R.A) was a prominent companion but not associated with leading the very first Sariya in this context. C is incorrect because Zaid bin Haris (R.A) was known for other services and is not credited with the first Sariya leadership. D is incorrect because Saad bin Abi Waqas (R.A) was a great general but not the one appointed for the initial Sariya.