So let's do this piece by piece:
A. The transcontinental railroad was completed on May 10th, 1869, meaning the transcontinental railroad was finished in that time. A is correct.
B. The population of the U.S. only started declining far later in its life, the idea of the 'land of opportunities' still represented America and population only grew. B is incorrect.
C. Reconstruction in the American south happened shortly after the civil war, but did not last a huge amount of time due to political strife and the rise of the ku klux klan. Reconstruction as a period ended in 1867, and the Reconstruction act ended shortly before. C is correct. (Sorry I don't know the date with this one.)
D. Again I do not know dates, however the gold rush as a whole ended in 1855, a decade before the beginning of this range. Hold would have been discovered earlier than this, and so we can say with confidence: D is incorrect.
Hope this helps!
