a) can come about much more readily if governments were to order the removal of the dams that limit the water-flow required by spawning fish.
hypothetical subjunctive for ( if i were to ...i would .. ) hold it as not sure about can
b) would come about much more readily if governments order the removal of the dams that limit the water-flow that spawning fish requires. --
if < present tense (order)> then would -- wrong
c) came about much more readily if governments would order the removal of the dams that limit the water-flow required by spawning fish.
if < would order > then came -- wrong
d) might come about much more readily if governments were to order the removal of the dams that limit the water-flow required by spawning fish.
same as A . and this option seems inferior to A .
e) would have come about much more readily if governments ordered the removal of the dams that limit the water flow that fish need in order to spawn.
if < past tense (ordered) > then only past tense or only would is allowed , but not would have -- wrong.
IMO A
Answer is D not A...
is not correct ...