Final answer:
b) first class engineer. A first class engineer's license would be required for a boiler plant with two 250 horsepower boilers connected to the same common header.
Step-by-step explanation:
The shift engineer's license required for a boiler plant with two 250 horsepower boilers connected to the same common header would be a first class engineer's license. A first class engineer's license is typically required for plants with multiple boilers or high capacity boilers. It demonstrates a higher level of expertise in managing and operating such systems.