Answer :
After the 4s subshell is filled, according to the Aufbau principle, the next subshell to be filled is the 3d subshell. So, the correct option is (b) 3d.
According to the Aufbau principle, after filling the 4s subshell, the next subshell to be filled is the 3d subshell. The Aufbau principle guides the order in which subshells are filled with electrons, showing that after neon, the electrons start filling the 3s subshell and after the 3p subshell is filled, it is followed by the 4s. Once the 4s is filled, the Aufbau principle indicates that the next electrons will go into the 3d subshell before the 4p subshell. This pattern is reflected in the structure of the periodic table and explains why elements in the middle section, known as the transition metals, have their 3d subshells filled after the 4s. So, the correct option is (b) 3d.
