Answer:

Arc welding is a commonly used welding process that involves the use of an electric arc to join metal pieces together. Carbon electrodes are often used in arc welding for various applications. When using carbon electrodes, a D.C. welding set with straight polarity is the most suitable equipment.

Explanation:

D.C. Welding Set with Straight Polarity:
- A D.C. welding set with straight polarity refers to a direct current (D.C.) welding machine that has a positive electrode and a negative workpiece connection.
- In arc welding with carbon electrodes, the carbon electrode acts as the positive electrode while the workpiece acts as the negative electrode. This configuration is known as straight polarity or direct current electrode positive (DCEP).
- Straight polarity is preferred when using carbon electrodes because it provides better control over the welding process and improves arc stability. It also reduces the electrode consumption and helps in preventing overheating of the electrode.

Other Equipment Options:
a) A.C. Welding Set:
- A.C. welding sets are not suitable for arc welding with carbon electrodes. A.C. welding sets provide alternating current, which is not ideal for maintaining a stable arc with carbon electrodes. The arc tends to flicker and become unstable, making it difficult to achieve good weld quality.

b) Rectifier:
- A rectifier is a device used to convert alternating current (A.C.) into direct current (D.C.). While rectifiers are used in some welding machines, they are not specifically designed for arc welding with carbon electrodes.

c) Motor Generator:
- A motor generator is a device that converts mechanical energy into electrical energy. While motor generators can be used to generate direct current for welding, they are not specifically designed for arc welding with carbon electrodes.

Therefore, the correct equipment option for arc welding using carbon electrodes is a D.C. welding set with straight polarity. This equipment provides the necessary control and stability required for successful arc welding with carbon electrodes.