I'm not sure what the greenest would be, but certainly walking or biking is greener than any motorized transport.
One can take issue with the study, but this comment doesn't seem to even grapple with it.
According to that, e-bikes aren't the greenest form of transportation. Walking, biking, and e-bikes came in at the same level of "greenness".
Although the paper itself points out that it's very likely overestimating the impact of bicycles, because the lifespan it assumes for them is too low.
if the electricity is supplied by solar power, it might be closest to the "greenest" form of transportation.