What are the insoluble impurities which settle down at the bottom of the anode known as? 

Anode mud

The correct answer is option 3. Anode mud.

During the electrolytic refining of metals, such as copper or silver, impurities often accompany the metal being refined. These impurities can be either soluble or insoluble in the electrolyte, depending on their chemical properties.

Insoluble impurities, along with other substances like oxides and other insoluble compounds, settle at the bottom of the electrolytic cell beneath the anode. This sediment, composed of various insoluble impurities, is collectively referred to as anode mud.

The formation of anode mud occurs because certain impurities present in the original metal feedstock do not dissolve in the electrolyte or do not participate in the electrochemical reactions occurring at the anode during the refining process. Instead, these impurities accumulate over time as solid particles, gradually settling at the bottom of the cell due to their higher density compared to the electrolyte.

The composition of anode mud can vary depending on the specific metal being refined and the nature of impurities present in the original metal. Anode mud often contains elements such as lead, selenium, tellurium, arsenic, and various other metals, depending on the impurities present in the starting material.

Anode mud is periodically removed from the bottom of the electrolytic cell and further processed to recover valuable metals or to dispose of it in an environmentally responsible manner.

In summary, anode mud refers to the solid sediment that forms at the bottom of the electrolytic cell during the refining of metals, containing insoluble impurities and other solid substances that do not dissolve or participate in the electrochemical refining process.