According to this,
n+l rule,
3d and 4p have same energy as
3+2 =5 4+1 =5
similarly, 4d and 5p have same energy.

b)

•  For subshells with the same value of (n+ℓ), electrons are assigned first to the sub shell with lower n. 
• A version of the aufbau principle known as the nuclear shell model is used to predict the configuration of protons and neutrons in an atomic nucleus.

Now we take a question to understand it clearly:

Q. n and l values of an orbital A are 3 and 2 and for other orbital B are 5 and 0. The energy of :

A) B is more than A

B) A is more than B

C) A and B are same

D) A is 4 times than B

Sol. Electrons are assigned to orbitals in order of increasing value of (n+l). For subshells with the same value of (n+l), electrons are assigned first to the subshell with lower n.

For orbital A,n = 3, l = 2, therefore, n+l=5

For orbital B,n = 5, l = 0, therefore, n+l =5

So, larger the n, greater is the energy.

So, the energy of B > energy of A.

c) (i)

• The exactly half-filled and fully filled orbitals have greater stability than other configurations. 
• The reason for their stability are symmetry and exchange energy. 
• The half-filled and fully-filled orbitals are more symmetrical than any other configuration and symmetry leads to greater stability.

(ii) 

• The half-filled and fully-filled orbitals are more symmetrical than any other configuration and symmetry leads to greater stability. 
• The electrons present in the different orbitals of the same sub-shell can exchange their positions. 
• Each such exchange leads to the decrease in energy known as Exchange Energy