(A)’s conclusion isn’t parallel, because it conditions the recommendation on a particular value (cost being the greatest concern). But the stimulus’s conclusion recommended a specific option without any condition.

The argument reasons by eliminating options. We have two options (make curtains or buy blinds). But one of those options is undesirable (buying blinds will take too long). So, we should go with the other option.

The options presented in (C) aren’t mutually exclusive (because we can do both), whereas in the stimulus, the options presented were.

(D) doesn’t contain a premise establishing that one of the options is undesirable. Although it does have a premise establishing that one of the options would make a particular goal difficult to achieve (buying fabric that matches what we already have), that doesn’t establish that this difficulty is so great that the option is undesirable. In addition, the conclusion asserts that we should go with a third option (buying blinds, instead of buying more fabric or making valances).

(A)’s conclusion isn’t parallel, because it introduces a third concept that wasn’t part of the initial choice between two options. In addition, it conditions its recommendation on not choosing one of the options. The conclusion in the stimulus did not condition its recommendation on anything.