When an active tooth in the shark’s jaws is lost or worn down, many spare teeth lie in seemingly limitless reserve, each of which are ready to slide into the appropriate position.

1. When an active tooth in the shark’s jaws is lost or worn down, many spare teeth lie in seemingly limitless reserve, each of which are ready to slide into the appropriate position.
2. Whenever an active tooth is lost or worn down, many spare teeth lie in seemingly limitless reserve in the shark’s jaws, which are each ready to slide into the appropriate position.
3. Many spare teeth lie in seemingly limitless reserve in the shark’s jaws, each one of which are ready to slide into the appropriate position when an active tooth is lost or worn down[tc1] .
4. The many spare teeth lying in seemingly limitless reserve in the shark’s jaws, each one of which is ready to slide into the appropriate position whenever an active tooth is lost or worn down.
5. In the shark’s jaws, many spare teeth lie in seemingly limitless reserve, each one ready to slide into the appropriate position whenever an active tooth is lost or worn down.

A should be the right answer.

Because "whenever" change the original meaning of "when", "ADE" is wrong

In C, each one of which are,

"single vs plural" does not match"!