B: The solid is a mixture.
An element would mean that only one kind of atom makes up the substance. This is proven to be false seeing that different particles have different melting points.
One extremely known example of a pure substance is water. Almost every one knows that it has a boiling point of 100 Celsius. Water is made up of two elements (hydrogen and oxygen). Although it is made up of two different elements, since they are bonded together as a pure substance, together they share one boiling/melting point. This is different from your mystery substance because certain parts melt at different temperatures while other parts remain solid.
The last option is incorrect because it is referring to boiling points instead of melting points like asked in the question. Plus it refers to three different temperatures when the question only tells you about two.
So, using the process of elimination, this is a mixture (B).
I hope that I helped you understand a little bit better. Have a good day.
