Which of the following is known as the Black lead? 

Graphite 

The correct answer is option 3. Graphite.

Graphite is often referred to as "black lead" due to its black or dark gray appearance and its historical association with lead. However, it's important to note that graphite and lead are distinct substances with different properties.

Here's an explanation of why graphite is known as "black lead":

1. Appearance:

Graphite is a form of carbon that has a black or dark gray color. Its physical appearance, especially in its natural, crystalline state, can resemble lead.

2. Historical Context:

In the past, before the distinction between different forms of carbon was well understood, graphite was sometimes mistaken for a form of lead. The term "black lead" was used to describe graphite due to this historical misconception.

3. Pencil "Lead":

The association between graphite and lead is also evident in the use of the term "pencil lead." The core of a pencil is made of a mixture of graphite and clay. The name "pencil lead" has persisted even though pencils contain no actual lead.

4. Graphite Properties:

Graphite is a crystalline allotrope of carbon with unique properties. It is a good conductor of electricity, has lubricating properties (as seen in pencil "lead"), and is chemically stable. These properties make it useful in various industrial applications.

5. Distinction from Lead:

Lead, on the other hand, is a different element with the symbol Pb (from the Latin word "plumbum"). Lead is a heavy metal, and its compounds are known for their toxicity. Due to health concerns, lead is not used in writing instruments or other applications where contact with humans is likely.

In summary, the term "black lead" historically refers to graphite, a form of carbon, and it highlights the historical confusion between graphite and lead, especially in the context of writing materials like pencils.