A student that just received their driver's license decides to buy a new car. He remembers from physics class that heat engines have a thermal efficiency that can
be calculated. He asks the car dealer for a car that has a high thermal efficiency
rating. The car dealer says he has a car that is 100% efficient.
Which law of thermodynamics best describes why the car cannot be 100% efficient?

measuring things 1 litre of water

you'd use the equation that establishes a relationship between  energy  and  wavelength, which looks like this

e=h⋅cλ, where

λ  - the wavelength of the photon; c  - the speed of light, usually given as  3⋅108m/s; h  -  planck's constant, equal to  6.626⋅10−34j s

plug the value you were given for the energy of the photon in the above equation to get the wavelength

e=h⋅cλ⇒λ=h⋅ce

λ=6.626⋅10−34js⋅3⋅108ms−1.257⋅10−24j=1.581⋅10−1m