Among 3d, 4s, 4p orbitals,
which one has least orbital energy ? Why ?

Explanation

4s' has least orbital energy. Because the energy levels of 3d, 4s, 4p are as follows.
`3d implies n + l implies 3 + 2 = 5`
`4 s implies n + l implies 4 + 0 implies 4`
`4p implies n + l implies 4 + 1 implies 5`
According to values of energies '4s' has least energy.