Answer C.
Below the highlighted text, the fact that in 1974-19997 real GDP per capita (g) grew 0.5% slower than it did during 1873-1973 may force us to believe that next few years g may be more slower...in further next few years it may become even more slower...and so on.... So there may be a one point in time in future when g becomes either constant or rather decrease !! resulting in g becoming lower than before (CONTRADICTING the premise or claim mentioned in the HIGHLIGHTED text).
So it is proved that the fact cited ( "One point these journalists....between 1873 and 1973.") in support of highlighted claim ("the real GDP...ever brfore.") does not support the claim. Hence C.

yeah agreed, saying something that is already known says nothing relevant. So C