Final answer:
Galvanized steel is the most suitable material for a kitchen exhaust duct due to its durability, corrosion resistance, and ability to withstand high temperatures and humidity levels.
Step-by-step explanation:
The most suitable material for a kitchen exhaust duct is galvanized steel. Galvanized steel is durable, corrosion-resistant, and able to withstand high temperatures and humidity levels commonly found in kitchens. It is also resistant to grease and other chemicals present in the exhaust air. Furthermore, galvanized steel is relatively easy to install and maintain.