Option to pay tax on installment is not under self assessment . Any Tax on self assessment must be paid in one go .
Option to pay tax on installment on in case the liability created by Authority during final assessment or Tax Audit , that to be only when granted by commissioner.
DRC 20 is form to pay tax in installments( only in above conditions) , refer section 80
Note : liability arising on monthly 3b cannot be paid in Installments