Combined, the machines fill one order in 15 minutes. In those 15 minutes, each of the machines fills some fraction of the order. Since we know the time (15 minutes), we need to know the rate of machine X to find the fraction of the order--how much of the job--machine X would fill.

Statement (1) is sufficient: if machine X fills one order in 60 minutes, we can find out how many orders it could fill in 15 minutes--one-fourth as many, or (1/4) of an order.

Statement (2) is also sufficient. Using the same technique, we can determine that Y would fill (3/4) of an order in 15 minutes, which leaves (1/4) of the order for machine X. Choice (D) is correct.