The correct choice is (d) Continuous process
Explanation: Amounts of mass entering and leaving the system are specified using flow rates, a mass balance based on rates is called a differential balance. Whereas, each term of the mass-balance equation in this case is a quantity of mass, not a rate. This type of balance is called an integral balance. Differential balances for continuous systems operating at steady state and integral balances for batch and semi-batch systems between initial and final states are used.