In this question, we are asked to identify which of the given options is not a mixture.

Option 1: Graphite

Graphite is a form of carbon that consists of only one element. It is not a mixture because it does not contain different substances mixed together. Therefore, option 1 is not a mixture.

Option 2: Glass

Glass is a solid material that is made by melting different substances together, such as silica, soda, and lime. It is a mixture because it contains multiple components mixed together. Therefore, option 2 is a mixture.

Option 3: Brass

Brass is an alloy made by combining copper and zinc. It is considered a mixture because it is composed of different substances mixed together. Therefore, option 3 is a mixture.

Option 4: Steel

Steel is an alloy made primarily from iron and carbon. Like brass, it is a mixture because it contains different components blended together. Therefore, option 4 is a mixture.

Based on the explanations above, we can conclude that the correct answer is option 1, Graphite, which is not a mixture.