How many orbitals in this subshell have an unpaired electron, if an atom has eight electrons in a $3d$ subshell?

The correct option is B $2$
The $3d$-subshell will contain a total of five $3d$-orbitals. When assigning electrons in orbitals, each electron will first fill all the orbitals with similar energy (also referred to as degenerate) before pairing with another electron in a half-filled orbital. The atom having eight electrons should have two $3d$ orbitals with an unpaired electron.

Hence, the correct option is $\text{B}$