I feel the answer must be E.
Conclusion: if income per capita keeps increasing while bean production cannot grow., Tyrannia will need to import beans, lard, or both
So there is a jump, over an assumption, from Red to blue.
1 pound of lard requires multiple pounds of beans.
Consumption of lard is increasing as Tyrannia's income per capita is also increasing.
Upon the flow, Tyrannia will need to import beans only if the people are not growing the beans AND these people have continued the production of lard, without having reduced the consumption of beans.
Considering all this random stuff, I feel that the answer must be E.
Whats the OA?
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C is wrong as it ONLY talks about lard consumption amongst the population. We don't know anything about beans consumption. Thus we can't tell if beans supply will go down ultimately. It could but since this is an assumption question, we need to prove that it MUST and this answer doesn't do that.

I ll try to answer your about why not C?
Conclusion: Therefore, if income per capita keeps increasing while bean production cannot grow, Tyrannia will need to import beans, lard, or both

Premise 1 : For years, Tyrannia has supplied itself with enough beans and lard by keeping demand below average.

Premise 2 : However, consumption of lard per capita is increasing toward average as Tyrannia’s income per capita is increasing toward average, and producing one pound of lard requires multiple pounds of beans.

An assumption is unstated part of the argument which the author takes for granted. Its already stated in premise1 that they keep the demand low as to meet the consumption.Than its stated that consumption is directly propoetional to per capita income and thus they have to import either or both because they are not able to keep the demand low. Author is adamant about importing the stuff hence i think the conclusion assumes that its not possible to reduce the per capita income or the consumption (which is totally out of scope as the author concludes that either or both needs to be imported). It directly leaves a gap that per capita income will not reduce that implies the consumption will not reduce.

Coming to C) In Tyrannia, consumption of lard per capita is practically equal across all income levels.
Try negating it consumption of lard is not equal across all income level, I dont think that it affect the conclusion in any way as the conclusion states about importing either or both.....

Hope that helps>>>>>>>>