Since the price of exports has reduced, export revenue will be less (in the short term). Consequently, aggregate demand will be reduced (AD = C + I + G + X - M), and so to counter this, the government can increase expenditure (G). (therefore, the answer is C)
Increasing interest rates and increasing the tax rate will have the opposite impact to what is necessary, lowering aggregate demand. (ruling out A and B)
Increasing the currency value might help, but it'll more than likely just hinder international competitiveness. (ruling out D)
That's my reasoning for C being right, anyone else have something different?