Correct answer:

1. Thermal burn: the commonest cause of a burn, due either to direct contact with a hot object or flames or to hot vapour such as steam.
2. Chemical burn: such as from caustic material. The chemical may penetrate deep into the skin and be difficult to remove.
3. Radiation burn: due to exposure by radiation, as in the local erythema that may follow superficial radiotherapy.
4. Electrical burn: severity depends on strength of current and the duration of contact.