This points out a separate advantage of barley compared to wheat, but it doesn't strengthen the specific hypothesis that the buildup of salt residues in the soil drove the shift from wheat to barley. Even if barley required less irrigation, and so led to less salt buildup in the future, this wouldn't get rid of the salt residues already in the soil from the excessive irrigation for wheat. We still want to know how that initial salt buildup related to the shift from wheat to barley.

This supports the idea that salt buildup drove the shift away from wheat and towards barley. Wheat struggled in soil with salt buildup, while barley was able to handle those conditions better.

This just describes the proportion of wheat to barley before 2900 B.C. It doesn't explain why there was a shift after 2900 B.C., or support the historians' explanation for that shift.

This actually weakens the author’s argument, because it provides a different reason -- the wheat blights -- for why wheat production decreased and there was a shift to producing barley.

This just gives further support for something we already know: that barley began to be produced much more after 2900 B.C. But this answer choice doesn't support the historians' hypothesis, because it doesn't tell us anything about what caused the change.