BF 1 - "While biofuels may be more environment-friendly" - author accepts this statement and doesn't contest/dispute in the argument.
BF2 - "in other regions the same crops would be impossible or extremely costly to grow" - it supports the conclusion
Conclusion - it is highly debatable whether such fuels will in fact present a significant cost advantage.
Support - For instance, many common crops used as resources for biofuels can be economically grown only in limited parts of the world
A. The first is the position taken by the argument (BF1 is not a position); the second provides evidence to support that position(BF2 doesn't supports that position). Incorrect
B. The first describes the circumstance that the argument as a whole seeks to establish(wrong- argument is not seeking to establish any circumstance given by BF1); the second opposes that established position. (wrong) - incorrect
C. The first offers evidence that the argument does not dispute (correct - as explained above); the second exemplifies the conclusion(correct - BF2 supports/exemplifies the support of the conclusion). Correct
D. The first is a claim that is called into question by the argument(BF1 is not called into question); the second provides a circumstance opposed by that claim. Incorrect
E. The first states a possibility that the argument denies(the argument doesn't denies the possibility of BF1.); the second is evidence that is accepted by the argument. Incorrect.
Answer C