Question

why is a skin graft necessary when cells are damaged by a third - degree burn?
to promote the repair of existing cells
to protect the area from infection
to prevent mutations
to replace damaged nerves

Explanation:

In third - degree burns, the entire thickness of the skin is damaged. Skin grafts provide a new layer of skin to protect the underlying tissues from infection, which is a major risk in severely burned areas. They don't primarily repair existing cells (as the damage is too extensive), prevent mutations, or replace damaged nerves.

Answer:

to protect the area from infection