Litres (L) is the best unit of measurement in a thermos as thermoses are fairly big. If it was in a glass however you would say milliletres (ml) as they are not as heavy as litres.
The most suitable units of measure for hot cocoa in a thermos are milliliters (mL), liters (L), and cups. Milliliters provide precision for smaller portions, liters are better for larger quantities, and cups are easy for everyday context. The choice of unit should depend on the amount of hot cocoa and the intended use or audience.
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