I arrived at D.
(1) gives the range of k between 6,000 and 500,000. We know that k must equal 5.1, 51, 510, 5100, 51000, 510000, 5100000, etc.; the only number in this set that falls in the given range is 51,000, n is 4. Sufficient.
(2) gives an exact value of k, which therefore gives an exact value of n. Note that the GMAT will not give you a statement which will contradict any information given in the problem description, which means that this will yield an integer value for n. For the curious, it is also n = 4. Sufficient.
D.
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