A scientist is testing an unknown sample of matter to determine its identity. He attempts to perform a chromatography experiment on the uniform sample but finds it to be inseparable. Next, he begins to experiment with the reactivity of the sample and finds that under no circumstances will the sample decompose into simpler substances.
What category of matter does the sample most likely belong to?

a) element
b) heterogeneous mixture
c) homogeneous mixture
d) compound