Identify the part labeled 6 on the microscope diagram.

Label 6 points to the upper part of the microscope, directly below the eyepieces (5) and above the revolving nosepiece (7) and objective lenses (8). This section houses the optical components that connect the eyepieces to the objective lenses.

This part is commonly known as the body tube or head of the microscope. It holds the eyepiece and connects it to the objective lenses.

A. Arm: The arm (labeled 1) connects the head to the base and is used for carrying the microscope.
B. Base: The base (labeled 4) is the bottom support of the microscope.
C. Body tube / Head: This matches the description and location of part 6.
D. Revolving nosepiece: The revolving nosepiece (labeled 7) holds the objective lenses and allows them to be rotated.

Therefore, the correct option is C.

The part labelled 6 is the Body tube / Head.

The final answer is $\boxed{\text{C}}$