If CAB is coded as 723 - 5 58 in a coded language then how will DAD be coded ?

As per the CAB code, A is coded as 5 and should be the same in case of A in DAD.

For C in the CAB, C is the third letter in the English Alphabet. 

So, 1) $$3^2=9$$

2) $$9^3=\ 729$$

3) 729-6= 723.

These 3 steps are followed to find the code of C.

If we cross check with B, the second alphabet,

1) $$2^2=4$$

2) $$4^3=\ 64$$

3) 64-6= 58- Which is how B is coded.

So, for D- the fourth alphabet, we have

1) $$4^2=16$$

2) $$16^3=\ 4096$$

3) 4096-6= 4090.

So, Option B is the correct one.