Blocking scheme B is the best, since the plots near the river might have different properties as the plots far from the river.

Blocking scheme A contains only plots near the river and no plots far from the river, the white block will then have different properties from the grey block.

Blocking scheme B contains 2 plots of each in the white block and also 2 plots of each in the grey block, then the properties of the plots will be roughly the same in the two blocks and thus this blocking scheme is better.