For a sufficient assumption question like this, we need to plug the gap in the argument to make it valid. Let's look at this argument.
We are told that Cummings supported individuals over regimentation. The author then claims this is inherently contradictory, since metaphor must have literal language, and literal language is regementation.
It seems like our author is saying that since metaphor inherently uses some form of regimentation, Cummings must have used regimentation. However, are we ever told that Cummings used metaphor? No! So, to fill the gap, we must know that Cummings used metaphor. B fits this perfectly.