D) If the composition of a sample is fixed, the sample is a pure substance.
Explanation:
A) It can be separated by using physical means, such as filtering.
Of course using physical means as filtration or crystallization we increase the purity of a compound, however sometimes impurities may remain dissolved in the solution or they may co-precipitate with the analysed sample.
B) The components in a pure substance do not have to be in definite ratios.
If the components in a substance do not have definite rations it means the the substance is not pure, it is a mixture of something.
C) If the composition of a sample varies, the sample is a pure substance.
If the composition of a sample varies, it means that different components in the substance are changing so it is a clue that the substance is not pure.
D) If the composition of a sample is fixed, the sample is a pure substance.
If the composition of a sample is fixed, and it can be determined and proven, the substance is pure.
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