Which of the following facts is true about the united states during the period between 1865 and 1915? a) the transcontinental railroad was completed. b) the population of the us declined. c) congress voted down the reconstruction act. d) gold was discovered in the mountains of west virginia.

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!