How it is determined that which sperm cell will fuse with the ovum and also what makes sperms mobile in semen? Kindly answer sir/mam.

It is a matter of chance which sperm will fuse with the ovum as any sperm is able to fuse with the ovum. They swim and reach to ovum and the sperm that gets success to penetrate the ovum fertilizes the ovum.
Flagella provide motility to sperms.