(A) If the upward trend in out-shopping continues at a significant level and the amounts paid by the government for Canadian social services are maintained, the Canadian goods-and-services tax will be assessed at a higher rate.
A continued trend in out-shopping means that lesser people will pay Canadian goods-and-services tax which will mean that government has to increase tax rate to be able to pay for the Canadian social services.
This falls within the boundaries of what is given in the premise and hence is the correct answer.
However there is an assumption here that the amount of shopping inside Canada has not increased enough to counter the out-shopping. I am going with this choice considering that the author has mentioned significant level for out-shopping. Also no other choice is even close to being correct.
(B) If Canada imposes a substantial tariff on the goods bought across the border, a reciprocal tariff on cross-border shopping in the other direction will be imposed, thereby harming Canadian businesses.
Nothing is given about taxes on other side so we shouldn't assume anything
(C) The amounts the Canadian government pays out to those who provide social services to Canadians are increasing.
The amounts provided for social services may increase because of increase in population or many other reasons and out shopping has nothing to do with it.
(D) The same brands of goods are available to Canadian shoppers across the border as are available in Canada.
Taking about Brand is out of scope.
(E) Out-shopping purchases are subject to Canadian taxes when the purchaser crosses the border to bring them into Canada.
Again out of scope. Nothing mentioned about the taxes on buying across the border.