Final answer:
The 2s orbital has the lowest energy.
Step-by-step explanation:
The orbital with the lowest energy is the 2s orbital.
The energy levels of orbitals are determined by their principal quantum numbers (n) and subshells. Within each energy level, the s orbital is at a lower energy than the p orbital. Thus, the 2s orbital has lower energy than the 2p orbital.
The 3s and 3d orbitals are in different energy levels, so comparing them is not straightforward. However, according to the Aufbau principle, which states that orbitals are filled in order of increasing energy, the 3s orbital is filled before the 3d orbital, indicating that the 3s orbital has lower energy.