The correct answer is A.
The Bland-Allison Act was approved by the US Congress in 1878, after overriding the veto imposed by President Rutherford B Hayes. This bill stated that the US Treasury should buy a certain quantity of silver that should be put in circulation in the form of "silver dollars". Ater the Panic of 1873, and the subsequent five-year recession, the decision of using both gold and silver as standard was taken in order to control the exchange rates of the dollar.
Options B and C refer to punctual silver purchases that were accomplished after the enactment of the abovementioned act. Option D refers to a posterior act that actually was enacted to amend the Bland-Allison former one.