When common salt is sprinkled on an earthworm, it dies due to osmotic shock. Osmotic shock occurs when there is a sudden change in the concentration of solutes on either side of a semi-permeable membrane, which causes a rapid movement of water.

In this case, the high concentration of salt outside the earthworm`s body creates a hypertonic environment. As a result, water from the earthworm`s body moves outwards, causing dehydration and disruption of the worm`s internal balance. The loss of water and disruption of internal processes ultimately lead to the death of the earthworm.

The other options mentioned in the question can be dissected as follows:

- Respiratory failure (option 2): Sprinkling common salt on an earthworm does not directly affect its respiratory system or the ability to breathe.

- Toxic effect of salt (option 3): While common salt can be toxic to certain organisms in high concentrations, it is not the primary reason why earthworms die when salt is sprinkled on them. It is the osmotic shock caused by the salt concentration that results in their death.

- Closure of pores of the skin (option 4): Earthworms do not have pores on their skin that can be closed. Their