Correct spelling, explanation: while trunk may seem like a completely English word, it’s, in fact, based on the Latin word truncus. It became modified over the years, and the spelling changed from soft c to k, so the trunk can function well as an English word.

Definition of trunk:
noun
1. the main body of a tree
We found a squirrel’s tree hollow in the trunk of the tree.
2. a person’s torso
I go to the gym three times a week to strengthen the muscles of the trunk.
3. a nose of an elephant
There was a famous elephant that was painted on canvas with its trunk.
4. a space for luggage at the end of a car
The family barely managed to fit all suitcases in the trunk.

Incorrect spelling, explanation: turnk is an incoherent word because it looks like a wrongly spelled turn, rather than the correct word trunk. It can’t be correct, because trunk is based on truncus, and here both the spelling and the pronunciation simply don’t fit the base word.