Not necessary, because (A) states something about “any nation with oil reserves.” The conclusion is specifically about the nations that said their oil reserves didn’t change in 1997. So the argument doesn’t have to assume anything about all nations with oil reserves (which includes the ones that didn’t say their oil reserves didn’t change).

Necessary, because this is what’s required to trigger the premises that allow one to conclude that a nation’s oil reserves went up or down. If (B) were not true — if it’s NOT likely that old oil fields were drained or new oil fields were discovered in most of the nations that said their oil reserves were unchanged — then we have no reason to conclude that those nations were likely to be wrong about what they said.

Not necessary, because although the author must assume that at least some nations had oil reserves rise OR drop, that doesn’t require the author to believe that a nation experienced BOTH a rise AND drop. Even if every country only had either a rise or drop, but not the other, that would still support a claim about changes in countries’ oil reserves.

Not necessary, because the author doesn’t need to assume that any of the nations that said their oil reserves didn’t change actually BOTH drained oil fields AND discovered new ones. The author just needs to assume that for most of those nations, oil fields were drained OR new field discovered.

Not necessary, because the idea of “obligation” is irrelevant. Whether a nation is obligated to to report oil reserve changes correctly doesn’t affect whether a nation’s statement about its oil reserves is incorrect. We care about the truth of the statement, not about whether the nations SHOULD be truthful.