Which subshell is filled after 3p in the order of electron configuration?

The correct answer is based on the order of filling subshells in electron configurations, which follows the Aufbau principle. According to this principle, electrons fill the lowest available energy levels first before moving to higher levels.

After the 3p subshell is fully occupied, the next subshell to be filled is the 4s subshell. This is because the 4s subshell is actually at a lower energy level than the 3d subshell, despite being part of a higher principal energy level (n=4). Therefore, it takes precedence and gets filled before moving on to the 3d subshell.

In contrast, the 3d subshell, while part of the third shell, has a higher energy than the 4s subshell and thus is filled after 4s. The 4p and 5s subshells come into play later in the electron configuration sequence, being filled after the 4s and 3d respectively. Hence, the filling of the subshells follows a specific order that reflects their energies, making 4s the right choice after 3p.