Final answer:
The correct answer is option B. Regulate the fuel and air supplies.
Step-by-step explanation:
Modulating combustion controls regulate the fuel and air supplies in a combustion system. They are responsible for adjusting the flow rate of fuel and air to maintain optimal combustion conditions.
By modulating the fuel and air supplies, the controls ensure that the ratio of fuel to air is at the proper level for efficient and safe combustion. This is done by adjusting valves or dampers that control the flow of fuel and air into the combustion chamber.