Answer: The correct answer is Option B.
Step-by-step explanation:
Valence electrons are defined as the electrons which are present in outermost shell of an atom. This is determined by the electronic configuration of an element.
For the given options:
Option A: Vanadium (V)
The atomic number of this atom is 23 and number of electrons that are present are 23. The electronic configuration of vanadium is:
![V:[Ar]3d^34s^2](https://img.qammunity.org/2018/formulas/chemistry/high-school/m7t9anzpvxfwbm4cyfupw0sf6ett4fa6us.png)
This element does not have 3 electrons in 4d-subshell.
Option B: Niobium (Nb)
The atomic number of this atom is 41 and number of electrons that are present are 41. The electronic configuration of niobium is:
![Nb:[Kr]4d^35s^2](https://img.qammunity.org/2018/formulas/chemistry/high-school/c6s8l3gm3wb6jwc9o3c2sg9zya8coixvta.png)
This element have 3 electrons in 4d-subshell.
Option C: Copper (Cu)
The atomic number of this atom is 29 and number of electrons that are present are 29. The electronic configuration of copper is:
![Cu:[Ar]3d^(10)4s^1](https://img.qammunity.org/2018/formulas/chemistry/high-school/jysv88zx3vbe9emvw5d32tnjyfw0mj15yb.png)
This element does not have 3 electrons in 4d-subshell.
Option D: Thallium (Tl)
The atomic number of this atom is 81 and number of electrons that are present are 81. The electronic configuration of thallium is:
![Tl:[Xe]4f^(14)5d^(10)6s^26p^1](https://img.qammunity.org/2018/formulas/chemistry/high-school/nz2kljdvv62h4k25kyvvqywr1cj8hq1mrg.png)
This element does not have 3 electrons in 4d-subshell.
Hence, the correct answer is Option B.