Final answer:
In a lap joint, complete penetration may not always be required, but complete fusion is essential to ensure a strong weld. Penetration relates to the depth of the weld, while fusion is about the melding of weld and base material.
Step-by-step explanation:
The question pertains to the characteristics required in certain types of weld joints. When referencing a lap joint, which is a specific type of welding joint, the statement suggests that one quality is not always necessary while another is essential for the joint's integrity. The correct answer is:
a. Penetration, b. Fusion.
Penetration refers to the depth to which the weld penetrates the base material, whereas fusion is the process by which the welding material melds with the base material to create a strong bond. In the case of a lap joint, complete penetration may not be necessary, yet complete fusion is required to ensure that the weld holds the two pieces of metal securely together.