Final answer:
The area of a rectangular room measuring 18ft by 21ft is 378 square feet, which corresponds to Option A: 378 ft².
Step-by-step explanation:
The area of a rectangular room can be calculated by multiplying the length by the width. For a room that is 18ft by 21ft, the area is:
Area = Length × Width = 18ft × 21ft = 378 ft².
Therefore, the correct answer is Option A: 378 ft².