Among 3p, 4s, 3d and 4p, the orbital having least energy is

According to Aufbau’s principle electrons are enter into orbitals as per increasing order of their energy values.

For 3p orbital, $\text{n}+l=3+1=4$

For 4s orbital,  $\text{n}+l=4+0=4$

For 3d orbital,  $\text{n}+l=3+2=5$

For 4p orbital,  $\text{n}+l=4+1=5$ 

Note: If two or more orbitals have same $n+l$ value then the orbital with smaller ‘n’ value has the lowest energy.  
Therefore, among the given all orbitals 3p has least energy.