Final answer:
The capacity of a large thermos is best measured in liters, which is a metric unit for measuring capacity suited for larger volumes of liquid.
Step-by-step explanation:
The best unit to measure the capacity of a large thermos would be liters (a). The capacity refers to the amount of liquid a container can hold, and liters are the appropriate metric unit for measuring larger volumes of liquid, such as in a thermos. While milliliters (mL) are used for smaller amounts, like cough syrup, liters are better suited for larger volumes, like a bathtub or a car's fuel tank. Options b (pounds), c (inches), and d (Kelvin) are not units of capacity; pounds measure weight, inches measure length, and Kelvin measures temperature.