Concept:Impurities weaken the structure of a pure solid, lowering its melting point.
Explanation:A pure solid has a regular lattice held by uniform intermolecular forces.
When an impurity is added, it disrupts this regular arrangement.
The disrupted regions require less energy to break apart.
Therefore, the solid melts at a lower temperature than the pure substance.
This effect is known as melting point depression.
The more impurity present, the greater the reduction in melting point.
Answer:The presence of an impurity causes the melting point to reduce.
Correct option: B. reduce.